Place bread cubes in a greased 13x9-in. baking dish. In a large skillet, cook and crumble sausage with pepper and onion over medium-high heat until no longer pink, 5-7 minutes. With a slotted spoon, place sausage mixture over bread. Sprinkle with cheese., In a large bowl, whisk together eggs, mustard and milk; pour over top. Refrigerate, covered, overnight., Preheat oven to 350°. Remove strata from refrigerator while oven heats., Bake, uncovered, until a knife inserted in center comes out clean, 30-35 minutes. Let stand 5 minutes before cutting. Freeze option: Cover and freeze unbaked casserole. To use, partially thaw in refrigerator overnight. Remove from refrigerator 30 minutes before baking. Preheat oven to 350°. Bake casserole as directed, increasing time as necessary to heat through and for a thermometer inserted in center to read 165°.