Boil the potato slices until fork tender. Drain the juice of the salmon into a measuring cup. Add milk until it reaches 2 cups. In a small pot, melt butter, whisk in flour then add the salmon and milk liquid mixture. Keep whisking for a couple of minutes. Add 1 cup of the cheese. Stir until well incorporated. Preheat oven to 355°
In a casserole dish, layer some of the potatoes, flakes of salmon, spinach, then some of the cheese sauce. Repeat the layer. Top with the last of the potatoes and remaining shredded cheddar cheese. Cover the pan with foil.
Bake it in the oven for 30 to 35 minutes.