Melt butter in a medium frying pan over medium heat. Add mushrooms, peppers and onion; sauté until tender.

While vegetables are cooking, whisk together eggs and water. Pour egg mixture over vegetables in the frying pan.

Cover and cook over medium heat, occasionally poking through the mixture to allow uncooked egg to flow to the bottom of the pan.

When bottom is cooked and top is almost set, finish cooking the frittata on the stove top by covering with a lid, flipping it in the pan, or cooking under the broiler.

If flipping, invert the frittata onto a plate, then slide it back into the pan to cook the other side.

Alternatively, place the frying pan under a preheated broiler until the top is cooked and slightly puffed, about a minute or two.