Preheat oven to 350°. Prepare Béchamel Sauce; keep warm.
Melt margarine in a large nonstick skillet over medium-high heat. Add shallots; sauté 1 minute. Stir in mushrooms, salt, pepper, and garlic; sauté 3 minutes or until liquid has evaporated. Set aside.
Combine breadcrumbs, 1/4 cup cheese, and 1 tablespoon chives. Stir well; set aside. Add 1 3/4 cups cheese to Béchamel Sauce; stir until cheese melts. Cook pasta according to package directions, omitting salt and fat. Drain well; return pasta to pan. Add mushrooms, cheese sauce, and 3 tablespoons chives; stir well.
Spoon pasta mixture into a 3-quart casserole; sprinkle with breadcrumb mixture. Bake at 350° for 30 minutes.