You throw the onion away at the end. That's the instruction — tomatoes, butter, an onion cut in half, forty-five minutes at a bare simmer, then the onion goes in the bin, having done all the work. Every kitchen I trained in would have minced it, sweated it, hidden it in there. Marcella Hazan was 89 when she died in 2013, and she was right and we were wrong.
She wrote those recipes in Italian and Victor translated them at the kitchen table, arguing, which is why the English has that spoken quality — someone saying it aloud to someone else. Judith Jones took the first book on at Knopf; she'd done the same for Julia Child, so she knew the sound of a cook who means it.
Those tomatoes come off the slopes south of Naples where the soil's old Vesuvius ash — free-draining, so the vines stay a bit thirsty and the fruit concentrates instead of watering down. Same reason the broom and the wildflowers up there smell stronger than they've any right to.