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