Melt butter in a saucepan. Set heat at medium. Then whisk melted butter with flour until you get a smooth paste.
Add milk. (You can also add the reserved liquid you got from peas). Stir until mixture becomes thick.
Carefully break salmon into pieces. Break apart every large piece and also take apart bones.
Add peas and onions to the salmon and stir. (Stir with a wooden spoon to avoid the peas from being mashed)Cook until thoroughly heated.
Add pepper for spicy taste and salt to taste.
Serve with desired side dish.