Melt the butter in a large saucepan. Add the onion and cook until golden, about 5 minutes. Stir in the flour until smooth. Gradually add the milk, stirring, until mixture boils and thickens, about 10 minutes; stir in the Parmesan-Reggiano cheese. Remove from heat.
Cook the pasta in plenty of boiling salted water 5 minutes; add the peas and carrots and cook until the pasta is firm to the bite, according to package directions. Remove from the heat and add 1 cup cold water to stop the cooking; drain. Add the pasta and ham to the cheese sauce. Transfer to a shallow 2-qt baking dish. Tuck the pieces of egg into the pasta mixture. Sprinkle with the shredded Italian Fontina cheese.
Bake in a preheated 350* F. oven until top is lighly browned, about 25 minutes.