To make the Vegetable Layer: In a large skillet over moderate heat, melt the 2 tablespoons of butter. Add the leek and saute for 2 to 3 minutes, or until softened. Add the bell pepper and suate for 3 minutes more. Add the zucchini and spinach and saute for 3 minutes more. Season to taste with 1/8 teaspoons each of the salt and pepper. Using a slotted spoon, transfer the mixture to a 2-1/2 quart casserole and set aside.
To make the Meat Layer: In a large skillet, heat the oil over moderate heat for 1 minute. Add the onion and celery and saute for 5 minutes, or until the onion is translucent. Add the beef and cook, stirring continuously, for 5 minutes, or until no pink color remains. Stir in the stock, tomato paste, oregano, thyme, and basil. Cook, uncovered, stirring occasionally, for 5 minutes. Season to taste with the 1/8 teaspoon each of the salt and pepper. Remove the pan from the heat. Spoon the meat mixture evenly over the Vegetable Layer.
To make the Macaroni-Cheese Layer: Bring a large saucepan of water to a boil over high heat. Cook the macaroni in the boiling water, or until al dente. Drain well, rinse under cold running water, and drain again.
Preheat the oven to 350*F. In a medium-size saucepan over moderate heat, melt the 2 tablespoons of butter. Slowly add the flour and cook, stirring continuously, for 2 minutes, or until a pale straw color. Gradually add the milk, stirring continuously, until the sauce is smooth. Bring to a boil, reduce the heat to low, and simmer, uncovered, for 2 minutes, or until thickened slightly.
Remove the pan from the heat and stir in 1-1/4 cups of the cheese. Season to taste with the white pepper. Stir in the macaroni. Spoon the macaroni mixture over the Meat Layer. Sprinkle with the remaining 1/4 cup of cheese.
Bake for 25 to 30 minutes, or until the top is golden and the layers are heated through. Remove the casserole from the oven and serve immediately.