Put steak on sheet of plastic wrap and place it in freezer for 20 to 30 minutes or til partially frozen and stiff. Slice beef against grain into slices 1/4 inch thick and 1/2 inch wide.
Heat oil in skillet over medium heat. Add green pepper and onion and cook,stirring frequently for 5 to 7 minutes or til vegetables soften. Add sliced beef and season to taste with salt and pepper. Saute beef for 1 minute or til lightly browned. Set aside
Preheat oven to3 75. Grease a 13 x 9 inch baking pan. Bring pot of salted water to a boil over high heat. Cook pasta til it is just beginning to soften to the al dente stage. Drain pasta, run it under cold water and return it to the pot.
Heat butter in pan over medium low heat. Stir in flour and cook, stirring constantly for 1 minute or til mixture turns slightly beige, is bubbly and appears to have grown in volume. Increase heat to medium and slowly whisk in warm milk. Bring to a boil, whisking frequently. Reduce heat to low, stir in nutmeg and simmer sauce for 2 minutes.
Add cheese to the sauce by 1/2 cup measures, stirring til cheese melts before making another addition. Pour sauce over pasta and stir well. Fold in beef mixture, season to taste with salt and pepper and transfer pasta to prepared pan. Sprinkle breadcrumbs on top of the dish.
Bake casserole for 20 to 30 minutes or til cheese sauce is bubbly and the crumbs on the top are deep brown. Allow to sit for 5 minutes, then serve. to make this in advance, or to reheat, cover with foil and reheat at 325. The amount of time depends on the amount you're reheating. You can also reheat in the microwave. Add a bit of milk and cover the microwave safe bowl with plastic wrap. Microwave n medium (40 percent power) and stir often.