Heat oil in a small, non-stick frying pan over medium-high heat. Add onions, fry until glistening with oil and slightly browned.
Toast bread until cooked to desired. Meanwhile, add spinach to mushrooms; fry until spinach has wilted.
Serve toast topped with butter, mushrooms, and spinach. Season with salt and pepper.
Try frying tomato halves, with the mushroom and spinach for an even bigger low protein breakfast!