First of all, prepare the salmon to make patties. Drain and save the liquid for later use. Then your salmon should be flaked with its skin and bones removed.
In a bowl mix the patty ingredients including lemon juice, mashed potatoes, grated onion, and beaten eggs. Also, sprinkle the black pepper over the salmon mixed with the rest.
Your patty mixture is ready. Make patties out of this mixture and then coat them in the flour before frying.
Further, it's the time for frying. Shallow fry the patties on medium heat and flip to fry from both sides. It will take about 10-12 minutes.
For making sauce
Defrost the peas. Melt some butter in a pan and add flour to it. Whisk well to get a smooth mixture and then add the salmon liquid to it.
Pour some milk and whisk to attain a creamy and saucy texture. Add the peas to the sauce to make them and turn the heat off in a few seconds.
Put the patties on the serving plate and drizzle the pea sauce over these patties.