Step 1: Pat salmon fillets dry with paper towels. Season generously with salt and black pepper on both sides.
Step 2: Heat olive oil in a large oven-safe skillet or cast iron pan over medium-high heat. Once shimmering, add 2 tablespoons of butter and let it melt.
Step 3: Place salmon fillets in the hot skillet, skin-side down if applicable. Cook for 4-5 minutes until the skin is crispy (if skin-on) and the salmon is cooked halfway up the sides. Carefully flip the salmon and cook for another 2-3 minutes, or until cooked through to your desired doneness. Remove salmon from the skillet and set aside on a plate, tented with foil to keep warm.
Step 4: In the same skillet, add the remaining 2 tablespoons of butter. Once melted, add the sliced mushrooms. Sauté for 5-7 minutes, stirring occasionally, until the mushrooms are tender and have released their liquid. Do not overcrowd the pan; cook in batches if necessary.
Step 5: Add the minced garlic to the skillet with the mushrooms. Sauté for 30 seconds to 1 minute until fragrant, being careful not to burn the garlic.
Step 6: Add the fresh spinach to the skillet. Cook, stirring frequently, until the spinach has completely wilted down, about 2-3 minutes. Season the vegetable mixture with a pinch of salt and pepper.
Step 7: Squeeze fresh lemon juice over the cooked mushrooms and spinach. Stir to combine.
Step 8: Return the cooked salmon fillets to the skillet with the vegetables. Spoon some of the garlic butter sauce and vegetables over the salmon. Garnish with fresh parsley or dill, if desired, and serve immediately. Enjoy your delicious Healthy Garlic Butter Salmon Spinach Mushrooms Weeknight Winner!