Heat butter and olive oil in a frying pan, add potatoes and brown on all sides. Transfer to a plate, then cook mushrooms on both sides for 5 minutes, adding a little more oil or butter if needed.
Transfer to a plate and return potatoes to the pan. Sprinkle in the garlic, then add the mushrooms and spinach.
Mix the eggs and milk together, season with pepper and pour into the pan.
Cover and cook gently for 5 minutes. Grill tortilla under medium heat for 6-8 minutes until golden on top. Check egg is set.
Serve hot or cold.
Fran Warde's cookbook Real Food For Families (Ryland, Peters & Small, £16.99)