Heat the vegetable oil in a pan and season the fillets with salt and chili flakes. Sear each side for about 5 minutes until golden brown and set aside to rest.
Sauté the onion until soft, then add garlic for a fragrant base. Stir in the mushrooms and cook until lightly browned to bring out their earthy flavor.
Whisk the cornstarch into the half & half and pour into the pan. Add Parmesan and stir until the sauce thickens to a creamy consistency.
Add the spinach and cook just until wilted. Season with lemon juice and additional chili flakes if desired. Return the salmon to the pan and coat with sauce before serving.