Preheat the oven to 210C/Gas 7.
1) In a 2-litre 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 the heat and stir in the nutmeg, 1/2 cup of Fontina cheese and prosciutto, and season with salt and white pepper. Set aside.
2) In a large pot, bring to a boil 5 1/2 litres 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 the pasta to the pot and pour in the bechamel sauce. Using a wooden spoon, mix well until all of the pasta is coated with the sauce.
3) Into a greased 23-by-33 cm baking dish, pour the pasta and cream sauce. Smooth out the top and sprinkle with the remaining 1/2 cup of Fontina. Dot the top with diced butter and bake in the oven for 25 minutes or until bubbling and the top is golden brown.