In a 2 quart saucepan, melt the butter over medium heat. Add the flour and whisk until smooth, about 2 minutes. Always stirring, gradually add the milk and continue to whisk until the sauce is smooth and creamy. Simmer until it is thick enough to coat the back of a spoon. This will take approximately 10 minutes. Remove from heat and stir in nutmeg, 1/2 cup fontina, prosciutto and season with salt and white pepper. Set aside.
In a large pot, bring to a boil 6 quarts of salted water. Add the rigatoni and cook for about 5 minutes. Since you will be cooking the pasta a second time in the oven, you want to make sure the inside is still hard. Drain in a colander. Return pasta to the pot and pour in bechamel sauce. Using a wooden spoon, mix well until all the pasta is coated with the sauce.
Into a greased 13-by-9-inch baking dish, pour the pasta with cream sauce. Smooth out top and sprinkle with remaining 1/2 cup fontina. Dot the top with diced butter and bake in oven for 25 minutes or until bubbling and the top is golden brown.