Peel and chop onions. Heat up a non-sticky pan with a dash of olive oil. Cook onions and leeks with a pinch of salt and pepper for 25 minutes stirring regularly and reserve.
Preheat oven to 200°C.
Cut 4 squares of aluminum foil to make your papillotes. Put a layer of leeks in each papillote. Place salmon filet on top. Pour some cream, add 1 pinch of salt and sprinkle with tarragon.
Put the 4 papillotes in an oven dish. Bake for 10 minutes and serve.