Peel and finely chop the onion. Heat butter in a pot. Sauté onion in it.
Add frozen peas, pour 75 ml water and lemon juice. Season with salt and pepper. Bring peas to the boil, cover and cook for 8-10 minutes.
Coarsely mash peas in vegetable water with a potato masher or roughly puree them with a hand blender. Stir in lemon peel. Season to taste with salt and pepper.