Peel the potatoes and clean the Brussels sprouts. Cut the potatoes into pieces and cook both for about 5 minutes. Add the sweet potato and cook for another 5 minutes. In the meantime, peel the onions, cut into small pieces and sauté with a little oil.
Heat the oven to 175 ° C top / bottom heat.
Put the steamed onions in a large, flat dish, and spread a layer of each of the potatoes, Brussels sprouts, sweet potatoes and halved cocktail tomatoes on top. Grate the parmesan and sprinkle over the layer. Layer the rest of the potatoes, Brussels sprouts, sweet potatoes, and cocktail tomatoes on top. Mix the whipped cream with the spices to taste and pour the sauce over the whole casserole, put semi-hard cheese on top.
Bake the casserole in a hot oven for about 30 minutes and let it brown for 5 minutes.