Unroll pie crust into 9" pie plate; flute edges. Prick bottom lightly with fork and freeze crust 10 min. In nonstick skillet, heat 1 tbsp. oil over medium-high heat until shimmering but not smoking. Season fish lightly with salt and pepper; sauté fish, skin side up, until it releases easily from pan (about 5 min.). Turn and cook until fish flakes easily with a fork and is opaque throughout (about 5 min.). Cool completely; discard skin and cut salmon into 1" chunks. Put a baking sheet in the oven; preheat to 350°. Meanwhile, in skillet over medium heat, heat remaining oil; sauté onion until softened (about 5 min.). Add spinach, cooking just until wilted (1-2 min.); set aside. In large mixing bowl, whisk eggs with milk and about 3/4 of the cheese; season with 1 tsp. salt and 1/4 tsp. pepper. Spread onion and spinach mixture in the bottom of crust; pour in egg mixture. Top with chunks of fish and sprinkle with remaining cheese. Bake on preheated sheet until filling is just set and pastry is golden (25-27 min.). Let stand 10 min. before cutting.