Peel potatoes; thinly slice. In a 3-quart saucepan combine potatoes, carrots, and bay leaves. Add water to cover. Bring to boiling; reduce heat. Cover and cook for 5 minutes.

Meanwhile, trim leeks, discarding the first 2 inches of the ends, and the first inch and outer leaves of the tough green tops. Wash leeks well. Add leeks to potato mixture. Cook, covered, 3 minutes or until vegetables are just tender. Drain well; discard bay leaves.

For sauce, in same saucepan melt 2 tablespoons margarine or butter. Stir in flour, salt, nutmeg, and pepper. Add evaporated milk. Cook and stir until thickened and bubbly. Cook and stir for 1 minute more. Remove from heat. Add 1/2 cup of the Gruyere cheese; stir until smooth.

Transfer the vegetables to a lightly greased 2-quart rectangular baking dish. Carefully spoon the warm cheese sauce over potato mixture; spread with the back of a spoon to coat the vegetables.

For crumb topping, in a small mixing bowl stir together grated Parmesan cheese, seasoned fine dry bread crumbs, and 2 tablespoons melted margarine or butter. Sprinkle the topping over the potato mixture.

At serving time, bake the casserole, uncovered, in a 350 degree F. oven about 15 minutes or until the cheese sauce is bubbly around the edges of the dish. Sprinkle with the remaining shredded Gruyere cheese. Bake about 5 minutes more or until the cheese is melted and the casserole is heated through. Makes 6 to 8 side-dish servings.

Prepare as above, except do not bake. Cover and chill in the refrigerator for up to 24 hours. Bake, uncovered, in a 350 degree F. oven about 25 minutes or until cheese sauce is bubbly around edges of dish. Sprinkle with remaining shredded Gruyere cheese. Bake about 5 minutes more or until cheese is melted and casserole is heated through.