Cook macaroni according to package directions; drain well, and set aside.
Melt 3 tablespoons butter in a large Dutch oven. Add lamb, onion, and garlic; cook, stirring frequently, until vegetables are tender. Drain off pan drippings; discard.
Add tomatoes, oregano, nutmeg, and cinnamon to Dutch oven. Simmer, uncovered, 40 minutes or until liquid has evaporated; cool.
Add 3 eggs, 1/2 cup Parmesan cheese, and macaroni, mixing well. Spread macaroni mixture into a lightly greased 13- x 9- x 2-inch baking dish.
Brush both sides of eggplant with olive oil. Place on a baking sheet; broil 5 minutes on each side or until golden brown. Layer eggplant slices over macaroni mixture; sprinkle with remaining 1/4 cup Parmesan cheese.
Melt remaining 3 tablespoons butter in a heavy saucepan over low heat; add flour, stirring until smooth. Cook 1 minute, stirring constantly. Gradually add milk, salt, and pepper, stirring constantly, until thickened and bubbly. Remove from heat.
Gradually stir one-fourth of hot mixture into 4 eggs; add to remaining hot mixture, stirring constantly. Add ricotta cheese; blend well. Pour over eggplant-macaroni mixture. Bake at 325° for 45 minutes. Let stand 10 minutes before serving.