PREHEAT oven to 375 F; BUTTER a casserole dish (*See note); SET aside.
PLACE torn bread crumbs in a medium mixing bowl.
In a small saucepan over medium heat, melt 1/4 of butter; POUR melted butter over bread crumbs; TOSS; SET aside.
In a saucepan over medium heat; WARM milk.
In a large, high-walled skillet or dutch oven over medium heat, melt remaining butter; when bubbly, ADD flour; COOK, STIRRING, one minute.
Slowly ADD warm milk to butter mixture, WHISKING constantly until mixture bubbles and thickens, about 8 to 12 minutes.
REMOVE pan from heat; STIR in salt, nutmeg, peppers, and about 2/3 of the cheddar and Gruyere; SET aside.
BRING a large pot of salted water to a BOIL; ADD macaroni; COOK until outside of pasta is cooked and inside is underdone, about 3 minutes.
In a colander, RINSE macaroni in cold running water; DRAIN well; STIR macaroni into cheese sauce.
POUR mixture into prepared casserole dish; SPRINKLE bread crumbs and remaining cheese over top.
BAKE until golden, about 30 minutes.
REMOVE from oven, let COOL on wire rack for 5 minutes.
NOTES ON SIZES WHEN DOING 12 SERVINGS (original recipe--adjust accordingly if recipe has been resized) 1. Cheddar cheese should equal about 4 1/2 cups when grated; Gruyere should equal about 2 cups when grated; may substitute 5 ounces (1 1/4 cups) grated Pecorino Romano cheese for Gruyere. 2. Use a 3-quart casserole for 12 servings; use a broad, shallow casserole for an extra-crusty version.
Per Serving (excluding unknown items): 472 Calories; 32g Fat (54.3% calories from fat); 25g Protein; 36g Carbohydrate; 1g Dietary Fiber; 101mg Cholesterol; 818mg Sodium. Exchanges: 2 Grain(Starch); 2 1/2 Lean Meat; 1/2 Non-Fat Milk; 4 1/2 Fat.