Wash zucchini, cut in half, remove seeds with a spoon.
Preheat the oven to 180 °.
Filling:
Sauté the onion and garlic in butter until translucent. Braise the tuna and tomatoes. Fold in half of the cheese. Season to taste with salt, pepper and nutmeg. Pour the mixture into the zucchini. Put this in a greased baking dish and sprinkle with the rest of the cheese.
Mix the broth with the crème fraîche, shredded canned tomatoes, salt and pepper. Pour this sauce into the baking dish.
Cook the zucchini in the oven for about 25 minutes.