Step 1: Pat the salmon fillets dry with paper towels. Season generously with salt and black pepper on all sides. This helps create a beautiful crust and ensures the seasoning penetrates the fish.
Step 2: Heat olive oil in a large oven-safe skillet (cast iron works wonderfully) over medium-high heat. Once shimmering, add the salmon fillets, skin-side down if applicable. Sear for 4-5 minutes until the skin is crispy and golden brown. If using skinless, sear for 3-4 minutes per side until a nice crust forms.
Step 3: Flip the salmon fillets and cook for another 2-3 minutes. Remove the salmon from the skillet and set aside on a plate, tented loosely with foil, to keep warm. Don’t worry if it’s not fully cooked; it will finish later.
Step 4: Reduce the heat to medium. Add 3 tablespoons of butter to the same skillet. Once melted, add the minced garlic and sliced mushrooms. Sauté for 5-7 minutes, stirring occasionally, until the mushrooms are tender and have released their moisture, developing a beautiful golden color.
Step 5: Add the fresh spinach to the skillet. If your skillet is very full, add it in batches, allowing it to wilt down before adding more. Cook for 2-3 minutes, stirring, until all the spinach has completely wilted.
Step 6: Pour in the vegetable or chicken broth and lemon juice. Bring to a gentle simmer, scraping up any browned bits from the bottom of the pan (this is called deglazing and adds incredible flavor). Stir in the remaining 1 tablespoon of butter until melted, creating a luscious sauce. Season the sauce with salt and pepper to taste.
Step 7: Return the salmon fillets to the skillet, nestling them among the vegetables and spooning some of the garlic butter sauce over the top. Cook for an additional 2-5 minutes, or until the salmon is cooked through and flakes easily with a fork. The exact timing will depend on the thickness of your fillets.
Step 8: Garnish generously with fresh chopped parsley. Serve immediately, spooning extra garlic butter sauce and vegetables over each salmon fillet.