Pat the steelhead trout dry with paper towels. Season both sides with paprika, garlic powder, salt, and pepper.
Heat a large skillet over medium heat. Melt the butter, then stir in the garlic paste and cook for 1–2 minutes until fragrant.
Place the trout flesh side down in the skillet. Cook for about 4–5 minutes without moving it. Flip and cook another 4 minutes, or until the internal temperature reaches 150 degrees. Remove from skillet and set aside.
In the same skillet, add the diced onion and cook until softened.
Pour in the heavy cream and whisk. Stir in the tomato paste, garlic salt, parsley, and additional salt and pepper to taste.
Add the fresh spinach and cook until wilted.
Return the trout to the skillet. Spoon the sauce over the top and reduce heat to low. Warm for 1–2 minutes.
Serve over cooked rice and spoon extra sauce on top.