Preheat oven to 375°F. In saute pan over med-high heat, cook bacon pieces and onion until onion is tender. Add mushrooms and saute until cooked through. Add in spinach until wilted (only takes a few minutes.) Remove from heat, drain grease. Salt and pepper to taste. Meanwhile in large mixing bowl, beat eggs and milk. Stir in cheese and bacon mixture.
Pour all into a greased 9x13 pan.
Bake for about 50 minutes, until top is golden and center is set. Let cool about 10 minutes before cutting.