Switch the salmon skin-side right down to a plate (it won’t be cooked via). Soften 2 tablespoons unsalted butter within the pan. Add the mushrooms and sear undisturbed till the bottoms are well-browned, about 3 minutes. Add the garlic, half the thyme leaves, and the remaining 1/2 teaspoon kosher salt and 1/4 teaspoon black pepper. Prepare dinner, stirring a couple of times, till the mushrooms are browned throughout and simply tender, about 3 minutes.