Preheat oven to 400 degrees. Place salmon on a baking sheet and add salt, pepper, butter, and a drizzle of lemon juice. Cook salmon for 10 minutes, or until it reaches a temperature of 145 degrees. While the salmon is cooking, clean the mushrooms, remove the stems, and scrape out the gills with a spoon. Brush the mushroom tops with olive oil, add salt and pepper, and heat them over medium heat in a pan on the stovetop until the tops are slightly browned and softened.