Grease an 8-inch square casserole dish. In a nonstick skillet, heat the oil and sauté the onion until soft. Add the chopped bell pepper and cook until soft. Add the spinach and chopped bacon, stir and briefly heat.
In a bowl, whisk together the eggs, milk, dry mustard, salt and peppers. Spread the bread cubes evenly over the bottom of the casserole dish. Spoon the veggie and bacon mixture on top, and spread evenly. Pour over the egg mixture, then sprinkle the grated cheese over the top. Cover and refrigerate overnight, or at least for 2 hours.
Preheat the oven to 350 degrees F. Bake the casserole until set, about 25-30 minutes. Let cool for 5 minutes before cutting.