In a big bowl, stir mayonnaise, milk and eggs together. Mix well.
Stir in salt, black pepper, sweet peppers, cheese and leafy greens.
Heat oil on medium heat. Fry garlic for 30 seconds. Add mushrooms and fry for 2 minutes.
Put heat on low and pour egg mixture into pan. Gently stir and scrape the bottom of pan with a wooden spatula for 1 minute for even cooking.
Put the lid on and cook on low heat for 15 minutes.
Take the lid off. The middle of frittata should be soft but not runny. Secure the lid back on while you flip the pan upside down. You will have your frittata on the lid. With the lid on one hand, place the pan back on heat with the other. Gently slide the frittata onto the pan. Cook on low heat for another 5 minutes, covered.
With the lid still on, flip the pan again. The frittata will be on the lid, right side up. Gently slide the frittata onto a plate.
Serve with your favourite Sunday fry-up!