Step 1: Pat the salmon fillets dry thoroughly with paper towels. Season generously with salt, black pepper, and paprika on both sides.
Step 2: Heat a large, oven-safe skillet (cast iron works well) over medium-high heat. Add 1 tablespoon of olive oil. Once shimmering, place the salmon fillets skin-side down (if applicable) in the skillet. Sear for 4-5 minutes until the skin is crispy and golden brown.
Step 3: While the salmon sears, melt 2 tablespoons of butter in a small microwave-safe bowl or saucepan. Add the minced garlic and sauté for 30 seconds until fragrant.
Step 4: Flip the salmon fillets. Pour the garlic butter mixture over the salmon. Add the sliced mushrooms to the skillet around the salmon. Cook for 2-3 minutes, stirring the mushrooms occasionally, allowing them to start softening and soaking up the garlic butter.
Step 5: Add the fresh spinach to the skillet, piling it around and on top of the salmon and mushrooms. Cover the skillet (or if your skillet isn't oven-safe, transfer everything to a baking dish now). Cook for 2-3 minutes, or until the spinach has wilted significantly.
Step 6: If your skillet is oven-safe, transfer it to a preheated oven at 375°F (190°C) and bake for another 5-8 minutes, or until the salmon is cooked through and flakes easily with a fork. The internal temperature should reach 145°F (63°C). If not using an oven-safe skillet, continue cooking on the stovetop over medium-low heat, covered, until the salmon is done.
Step 7: Squeeze fresh lemon juice over the entire dish and garnish with fresh parsley or dill before serving.