For the meat sauce, put some olive oil in a large pot; saute onion until lightly browned; add garlic; add meat and cook until brown, crumbling with a fork; add remaining meat sauce ingredients to skillet; mix well; cover and simmer for 1 hour, stirring occasionally, until quite thick; let mixture cool; remove and discard cinnamon stick and bay leaf.
Prepare pasta by boiling until al dente in plenty of boiling, salted water; drain and place pasta in a large bowl; add butter, milk, eggs and 1/2 cup cheese; gently stir to coat; set aside.
To prepare bechamel sauce, melt butter over medium heat in a saucepan; stir in flour until smooth, cook, whisking for 2 minutes; gradually add milk, stirring, until thickened; lower heat; add salt, pepper and nutmeg; remove from heat.
In medium-sized bowl, beat 3 eggs; slowly add hot cream sauce to eggs; stir thoroughly so eggs don't curdle.
Preheat oven to 350°F and butter or oil (with olive oil) a large baking pan (my pan is about 18" x 10").
Empty 1/2 of the penne mixture over bottom of pan; cover evenly with all of meat sauce; sprinkle 1/3 of the cheese over meat sauce; add remaining penne mixture, spreading evenly; sprinkle another 1/3 of the cheese over top; pour bechamel sauce evenly over entire casserole; top with remaining cheese.
Bake until bechamel sauce sets and has golden-brown patches across top, approximately 45 minutes to 1 hour; cool at least 20 minutes before cutting into pieces for serving.
Freezer Notes: Once the baked pastitsio has completely cooked, cut serving-size pieces, double-wrap in foil and freeze. Defrost in the usual manner and reheat in microwave (or oven) for a delicious, quick meal, when there's little time to cook and dinner is ready and waiting in the freezer!