Sprinkle croutons, cheese, onion, peppers and mushrooms into two greased 8-in. square baking dishes. In a bowl, combine the eggs, milk, salt, mustard and pepper. Slowly pour over vegetables. Sprinkle with bacon.
Cover and freeze one casserole for up to 3 months.
Bake the second casserole, uncovered, at 350° for 45-50 minutes or until a knife inserted near the center comes out clean.
To use frozen casserole: Completely thaw in the refrigerator for 24-36 hours.
Remove from the refrigerator 30 minutes before baking. Bake, uncovered, at 350° for 50-60 minutes or until a knife inserted near the center comes out clean.