1package(10 oz.) frozen chopped spinach, thawed and moisture squeezed out
Cook pasta according to package directions. Drain; set aside.
Preheat oven to 350°F and lightly grease (or spray with nonstick cooking spray) a 1 ½ quart casserole or baking dish.
While pasta is cooking, heat oil in a skillet over medium high heat. Add mushrooms and onions, sprinkle with salt and pepper. Sauté for 6-7 minutes or until mushrooms are golden and onion is soft. Add garlic and cook, stirring, for 1 minute more. Remove from heat.
Combine egg, 1 cup Monterey Jack cheese, sour cream, milk, 2 tablespoons Parmesan, pepper, and nutmeg in large bowl. Add spinach, mushroom mixture, and cooked spaghetti; mix well.
Place in prepared casserole dish. Sprinkle with remaining Monterey Jack and Parmesan cheese.
Bake, covered, for 25 minutes; remove cover and bake for another 20 minutes or until bubbling and golden brown. If desired, check internal temperature with instant read thermometer inserted in center of casserole; it should read 165°F.
If desired, substitute fresh spinach for frozen. One pound of spinach equals one 10-ounce box. Sauté fresh spinach until leaves have wilted.
Any type of pasta can be substituted, including whole wheat.
Casserole can be made ahead and refrigerated overnight or frozen. Bake as directed, adding 10 minutes to covered portion of baking time.