Cut broccoli into small florets. Slice onion and mushrooms thinly.

Cut potatoes into bite-sized pieces, soak in water, drain, and microwave for 5 minutes until soft. Mash and mix with mashed potato ingredients.

Pat salmon dry, cut into bite-sized pieces, season with salt and pepper, and coat with flour.

Heat oil in a pan over medium heat, cook salmon until browned on both sides, then remove.

Wipe the pan clean, add butter, and sauté broccoli, onion, and mushrooms until softened. Add flour and cook until no longer powdery, then add remaining white sauce ingredients and stir until thickened.