Peel and finely chop the onions and garlic. Bring a good 1 l water to the boil and dissolve the stock in it. Keep the stock hot.
Heat 2 tablespoons of butter in a wide saucepan. Fry half onions and half garlic. Add rice and fry until transparent. Add wine and simmer at low heat while stirring until it evaporates.
Add so much hot stock until the rice is covered. As soon as the rice has absorbed the broth, pour in some broth again and again. Simmer open at low heat for about 25 minutes, stirring from time to time so that nothing sticks.
The risotto should be creamy, but the rice should still be somewhat firm to the bite.
In the meantime clean, wash and dice the vegetables. Wash sage, shake dry, pluck off leaves and chop finely. Heat oil in a pan with lid. Fry the bell peppers, zucchini, sage, remaining onions and garlic for 4-5 minutes.
Add the tomatoes and fry briefly. Season with salt and pepper. Cover and stew for about 5 minutes. Season to taste with salt and pepper.
Preheat oven (electric cooker: 225 °C/circulating air: 200 °C/gas: see manufacturer). Grate Parmesan cheese. Stir mascarpone and parmesan, except for 1 tbsp, into the finished risotto. Season to taste with salt and pepper.
Place the risotto in a greased ovenproof dish. Spread the ratatouille over it. Sprinkle with the rest of parmesan. Bake in a hot oven for about 5 minutes.