There is nowadays a genre of memoir that relies on examining old documents and retracing family footsteps, sleuthing out difficult truths from the past, which are then interleaved with the writer’s own experiences. Edmund de Waal’s The Hare with the Amber Eyes is the most obvious of those, but you could also count Hadley Freeman’s acclaimed House of Glass and Ariana Neumann’s When Time Stopped among them.

Then there are others that take a completely different route. Ignoring the siren call of facts or the temptations of traversing Europe in pursuit of historical research, they stay at home to delve deeply into the author’s own psyche. No less detective-like and requiring just as much hard graft, these involve the author excavating his or her own memories, forgotten moments and suppressed emotions, to unravel what really happened all those years ago. That is largely the approach of Denis Hirson’s powerful new book, and at times it feels as if one is sitting on the edge of the therapist’s couch alongside him.

Video calls with somebody you don’t know are always awkward, all the more so when one feels that one has been privy to the other person’s soul-baring. When I “meet” Hirson on a flickering WhatsApp connection to his home just outside Paris, there’s no chance to make a cup of tea and get comfortable in each other’s company. But alongside his tanned, craggy features and a receding halo of white hair, he has a very South African directness, which means that he seems comfortable getting straight down to business.

The key event of My Thirty-Minute Bar Mitzvah takes place in apartheid South Africa in 1964, where 13-year-old Denis is being raised by parents who are at best ambivalent about Jewish traditions. At the time, all he feels about the titular and rather unconventional episode is confusion, when it’s over he pushes it to the back of his mind. He tells me it was his daughter’s bat mitzvah many years later that “ricocheted back emotionally for me to the event of my 13th birthday”. Suddenly, “I was in tears before an audience of more than a hundred people…. and the past came surging up.” He says that the moment provided the “fuel” for this new book, though actually unravelling “the knot in my heart” and getting it down on paper was a painstaking process that took nearly five years. The memoir ponders what exactly Jewish faith means to Hirson, trawling back over key events in his childhood. He recalls hearing the shofar at shul with his Litvack grandfather, spending time with family in Israel and celebrating Passover. His parents even have the Hebrew names Baruch (meaning “blessed”) and Yael (“mountain goat”). Yet despite all that he is left feeling not Jewish enough, something distilled in a moment when he shares his packed lunch with a schoolfriend, only to have the proffered sandwich rejected as not kosher.

While other boys prepare in the time-honoured way for their bar mitzvahs, mastering the intricacies of Hebrew and the Bible is not required of Denis and he senses a vacuum in his life. Being Jewish “was a secret that I myself couldn’t crack” he writes. It’s something that still seems to beguile and baffle him. “It is fundamentally part of who I am,” he tells me, yet “full of contradictions.” Though the Hirsons led a more threadbare lifestyle than other Jews in their social circle, young Denis is all too aware that he is relatively privileged. His conflicted feelings about growing up white in an authoritarian, racially segregated country are something he has written about in previous memoirs, essays and poetry, as though the experience has etched such deep grooves into his soul that he feels drawn back to it over and over, in an attempt to resolve a trauma.

“South Africa was a very, very complicated place to grow up in,” he says now. “If one was white, if one had a maid, as we did like all other whites, she was then the living example, the living person in our house who was actually suffering from the system while we were benefiting from her presence.”

The central driver of the narrative is Hirson’s relationship with his father, and the slowly dawning realisation that besides being a physics lecturer, his parent is also a radical activist. While Baruch Hirson is often closeted in dark rooms, in the world outside the Sharpeville massacre of 1960 looms over everything. In spring of that year police in Transvaal province opened fire on a crowd of black demonstrators, killing 69 and injuring 180. The shooting was a watershed in the history of South Africa, prompting condemnation and increasing isolation abroad at the same time as escalating repression at home.

The political crackdown leads directly to the key turning-point in young Denis’s life – while he is away on a school choir trip, his father is arrested, charged with sabotaging electricity pylons and sentenced to nine years. For the son it is a catastrophe. He feels himself “sinking down, watching the world around me melt at the edges”, and is plunged into deep depression. Looking back, he reflects that politics at the time meant nothing to him but personal loss. I sense a visceral, ongoing anger with his father, who died in 1999, but Hirson disagrees. “I don’t think there’s only anger. I think there is love too. I don’t make any bones about the fact that my father was the one person I loved more than anyone else in the world. And I don’t think that expressing anger is a negation of love. I think that on the contrary, expressing it would be part of one’s openness to the other person, part of one’s ability to fully express oneself.”

Denis Hirson left South Africa in 1973, settling in France where he married and raised his family. He is the long-distance South African of his best-known poem, immersing himself in the literature of his homeland from afar.

He’s happy to admit he has chosen the life of a perpetual foreigner, seeing marginal status as essential to a writer, and distance from his subject just as vital. “It’s only I think, if one takes a step back and looks at it from the outside, that the deeper contradictions appear.” In the book, Hirson recalls a year spent in England before his father’s arrest, when the family visited many political exiles. With their yellow grass mats and the smell of the veld, he observes that “none of them had really left South Africa.” He could almost be writing about himself.

My Thirty-Minute Bar Mitzvah is published by Pushkin Press​