If you need to start on this recipe even earlier than the evening before your brunch, you can cook the sausage, onions, mushrooms, and garlic earlier in the day or a day ahead, and also chop the green peppers and green onions. Refrigerate the sausage mixture and the chopped peppers and onions in separate containers until assembling the casserole.
After the casserole is baked, you can refrigerate leftovers and reheat individual portions in the microwave for quick breakfasts all week.
This also freezes wonderfully. Pre-portion leftovers, wrap each portion individually in plastic, and store in a zipper container in the freezer. Microwave on several layers of paper towel to reheat as needed. After being frozen, the veggies let off water when reheated, but this casserole is still surprisingly terrific, so don't let that liquid worry you.
For Breakfasts On the Go (Or Snacks and Dinners on Busy Evenings): Nestle re-warmed slices of the casserole into a tortilla for a wrap.
Breakfast for Dinner: You can make this in the morning and refrigerate until baking, or prep it in the afternoon and then cook it immediately, for a fun "breakfast for dinner" option.