How to Make It
Preheat the oven to 350°. In a large pot of boiling water, cook the pasta until al dente. Drain well.
In a large, deep skillet, heat the olive oil. Add the onion and cook over moderately high heat, stirring occasionally, until softened, about 4 minutes. Add the ground lamb and veal and cook, stirring to break up the lumps, until the meat is beginning to brown, 10 minutes. Remove from the heat and stir in the diced tomatoes, tomato paste, marjoram and parsley. Season with salt and pepper. Transfer to a large bowl.
In a large saucepan, melt the butter. Add the flour and cook over moderate heat, whisking, for 2 minutes. Gradually whisk in the milk and cook over moderately high heat, whisking constantly, until the sauce is very thick and boiling, 7 to 8 minutes. Remove from the heat and whisk in the egg yolks and the Parmigiano-Reggiano. Stir all but 1 1/2 cups of the sauce into the meat mixture along with the pasta and season with salt and pepper. Spoon the pasta into a 3-quart baking dish. Spread the reserved 1 1/2 cups of sauce on top.
Bake the pasta in the center of the oven for about 30 minutes, until bubbling. Preheat the broiler. Broil the pasta 8 inches from the heat for about 2 minutes, until the top is browned and bubbling. Let rest for 10 minutes before serving.