Cook the potatoes in the oven or microwave. Process the potatoes through a food mill.
Place the mixture in a bowl with a hole in the center. Add the egg, the yolk the cheese, salt, pepper, nutmeg.
Gradually fold the flour and baking powder taking care not to overwork the dough. Roll out a piece of dough into a cylinder and cut out 1 inches pieces. Press the gnocchi using a gnocchi board or fork to form lines.
Bring a pot of water to boil with a pinch of salt. Let the dumplings cook for about 2 to 3 minutes. Drain off the water.
Rub an iron skillet (or small iron skillet as I did) with the crushed garlic clove.
Add the cooked gnocchi. Add the cream and sprinkle with the 3 types of cheeses and place in the oven at 350 F and the gratin dish.