Put the milk in a large saucepan with a pinch of salt and heat until you can see steam rising from it. Whisking all the time, sprinkle in the semolina in a slow and steady stream. The mixture will thicken and become hard to whisk. When all the semolina is in, lower the heat to medium-low and start to beat the mixture with a wooden spoon. Cook until it starts to leave the sides of the saucepan and is very thick. Take the pan off the heat. Stir in half the butter and all the cheese until they are completely incorporated, then add the egg yolks one at a time, stirring until they are mixed in too. Taste the mixture for seasoning.

Spread the mixture out in a medium (about 30 x 20cm) baking tin to the thickness of about 1.5 cm, making it as smooth as possible. Cover and when cool, put in the fridge to firm up – it will take about 45 mins-1 hr.

For the sauce, tip everything into a saucepan and season well. Simmer gently for 45 mins, stirring every so often. Heat oven to 220C/200C fan/gas 7.

Butter a gratin dish or baking dish (about 30cm wide). Using a pastry cutter or the rim of a glass, cut out rounds of the gnocchi mixture about 4cm across and lay them, slightly overlapping, in the dish. Melt the rest of the butter and pour it over the top, then scatter a generous amount of parmesan on top. Bake in the oven for about 25-30 mins or until the gnocchi are sizzling on top and golden.

Take the onion out of the tomato sauce, check the sauce for seasoning and serve with the gnocchi.