I created this casserole for my dad, who in recent years has cut back on starchy breakfasts in favor of those rich in protein and vegetables. He is picky about the vegetables he likes, and these are a couple of his favorites. The spaghetti squash blends into the eggs to the point that you barely know it’s there; it just adds a gentle sweetness and some bulk. If you prefer, you can leave it out.
Potluck Prep: You can roast the squash and garlic and cook the mushrooms up to 2 days in advance. Refrigerate them in separate containers. Then, you can refrigerate the assembled, unbaked casserole the night before and bake it in the morning. The casserole can stand at room temperature for up to 4 hours.