For the bechamel sauce: Melt the butter over medium-high heat in a saucepot, and then whisk in the flour 1 to 2 minutes. Whisk in the milk and season with some salt, pepper and a little nutmeg. Cook until the sauce thickens enough to coat a spoon and adjust the seasoning once more. Pour a ladleful of the sauce into a bowl with the yolks, whisking to temper the eggs before adding into the pot. Stir in the cheese and remove from the heat.
For the spicy meat sauce: Heat 2 turns of the pan EVOO in a large Dutch oven over medium-high heat. Brown the lamb and sausage well, breaking up the meat as it cooks. Stir in the bay leaf, garlic, onions and peppers and sprinkle with the rosemary, some salt and pepper. Cook until the onions soften, 5 minutes. Stir in the tomato paste and cook until fragrant, then pour in the wine and reduce by half. Add the stock and tomatoes, breaking them up as they cook, and reduce the heat to a simmer. Throw in the citrus rind and cinnamon stick to mellow the sauce, and simmer until the tomatoes cook down, 30 minutes. Discard the rind, cinnamon stick and bay leaf.
For the pasta: Bring a large pot of salted water to a boil and cook the pasta to al dente. Just before draining the pasta, add 1 cup of the starchy cooking liquid to your sauce. Beat the egg whites until frothy and combine with the cheese, and then toss with the pasta.
For assembly: Preheat the oven to 350 degrees F. Grease a 9-by-13-inch baking dish with EVOO. Spoon in half of the pasta, then layer with all of the meat sauce followed by the remaining pasta. Pour all of the bechamel on top as the last layer. (Cool and store for a make-ahead meal, bringing to room temperature before baking and serving.) Set the pastitsio on a baking sheet to catch any drips and bake until deeply golden on top, 1 hour. Cool 15 to 20 minutes, then cut and serve.
When purchasing the canned tomatoes, look for D.O.P. on the label. If tomatoes are in season, you can substitute 10 to 12 fresh ripe plum or Roma tomatoes, peeled and chopped.