Pat the salmon fillets dry with paper towels and season both sides with salt and freshly ground black pepper.
Heat the 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 the salmon is cooked through. Remove the salmon from the skillet and set aside on a plate.
Reduce the heat to medium and add 2 tablespoons of butter to the same skillet.
Add the minced garlic and sauté for 30 seconds until fragrant.
Add the sliced mushrooms and cook, stirring occasionally, for 5-6 minutes until the mushrooms are tender and browned.
Season the mushrooms with salt and pepper to taste.
Add the remaining 2 tablespoons of butter and lemon juice to the skillet, stirring until the butter is melted and the sauce is combined.
Return the salmon fillets to the skillet and spoon the garlic butter mushroom sauce over the top.
Sprinkle the chopped fresh parsley over the salmon and mushrooms.
Remove from heat and serve immediately.