Bring the milk, water, butter and garlic to a boil in a large pot; season with salt.
Remove pan from heat and whisk in the semolina. Place pan back over low heat and stir with a wooden spoon until it is smooth and creamy, about 10 minutes.
Remove pan from heat and stir in 1/4 cup of the Parmesan. Then quickly stir in the eggs. Pour onto a lightly greased sheet pan and spread with a spatula into a 1/2-inch-thick mass. Place the sheet pan in the refrigerator UNCOVERED and chill for at least 3 hours or overnight. If you cover the pan the semolina will remain loose and not firm up enough to cut. Once the semolina is thoroughly chilled it can be covered.
Preheat oven to 400° F. Position an oven rack in the top third of the oven.
Grease a 12-inch round ovenproof baking dish with butter and set aside.
Use a 1 1/2-inch round pastry cutter to punch circles out of the semolina. Layer in concentric circles, overlapping each round by half until the semolina is used up. Semolina scraps can be pressed together, chilled and cut again.
Pour the cream over the semolina gnocchi and sprinkle with the remaining 1/4 cup of Parmesan. Bake until bubbly and lightly golden, about 20 minutes. Shave truffles over the top and serve immediately.
For a variation, fold 1 cup cooked, finely chopped spinach or wild mushrooms into the semolina at the end of cooking.