Preheat oven to 350°. Lightly oil a deep pie pan or 2-qt baking dish. Melt butter in a saucepan; add onions and sauté until softened. Sprinkle flour over onions and stir to combine. Cook a minute or two and then add milk, stirring constantly. Cook and stir until thickened. Reduce heat to a simmer and stir in mustard, peas, salmon, salt and pepper and cook a few minutes longer. Remove from heat and add parsley and Cheddar cheese. Spoon this mixture in prepared pan. Spread mashed potatoes on the top, using a fork to draw up peaks. Brush with egg white if desired for a crustier finish. Bake for 25 minutes until golden.