Preheat the oven to 325 degrees F (165 degrees C).
Combine olive oil, garlic seasoning, chives, lemon juice, garlic, salt, and pepper in a medium bowl until well blended.
Place salmon on a baking sheet and coat with the garlic mixture; reserve any remaining mixture.
Bake in the preheated oven until fish flakes easily with a fork, 20 to 30 minutes.
While salmon bakes, heat a large skillet over medium to medium-high heat. Add mushrooms and the rest of the garlic mixture. Cook and stir until mushrooms shrink slightly, 3 to 5 minutes. Add butter; cook and stir until mushrooms are mostly tender, 2 to 3 minutes more.
Set an oven rack about 6 inches from the heat source and turn the oven to broil. Add mushrooms to the baking sheet with the cooked salmon and broil until golden brown, 5 to 10 minutes.
