Peel, thinly slice potatoes.  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.

Add green onions to potato mixture.  Cook, covered, 3 minutes or until vegetables are just tender.  Drain well; discard by leaves. 

For sauce, in same saucepan melt 2 tbsp 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 Swiss cheese, stir until smooth.

Transfer the vegetables to a lightly greased 2 quart 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 tbsp melted margarine or butter.  Sprinkle the topping over the potato mixture.

Bake the casserole, uncovered, in a 350° oven about 15 minutes or until the cheese sauce is bubbly around the edges of the dish.  Sprinkle with the remaining shredded Swiss cheese. 

Bake about 5 minutes more or until the cheese is melted and the casserole is heated through.