Pat the salmon fillets dry with paper towels and season both sides with salt and black pepper.
Heat olive oil in a large skillet over medium-high heat.
Place the salmon fillets skin-side down in the skillet and cook for 4-5 minutes without moving them, until the skin is crispy.
Flip the salmon fillets and cook for an additional 3-4 minutes until cooked through. Remove salmon from the skillet and set aside on a plate.
Reduce heat to medium and add 2 tablespoons of butter to the same skillet.
Add the minced garlic and sauté for about 30 seconds until fragrant.
Add the sliced mushrooms, season with a pinch of salt and pepper, and cook for 5-6 minutes until the mushrooms are tender and browned.
Add the remaining 2 tablespoons of butter to the skillet and stir until melted, coating the mushrooms in the garlic butter sauce.
Return the salmon fillets to the skillet and spoon the garlic butter sauce and mushrooms over the top.
Drizzle lemon juice over the salmon and sprinkle with chopped fresh parsley.
Remove from heat and serve immediately.