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.
Top Tip:
Try frying tomato halves, with the mushroom and spinach for an even bigger low protein breakfast!