Bring a pan of boiling water to boil and cook the spaghetti according to the packets instructions.
Meanwhile, heat the butter and fry the onion for 5 minutes or until it has softened.
Add the peas, salmon, crème fraiche and 50ml of water. Season with salt and pepper. Cooking for 3-4 minutes until cooked through.
Drain pasta and stir in the sauce till it coats the pasta. Serve hot.