Set an Instant Pot to sauté on high. Add 2 tablespoons butter and 4 ounces sliced mixed mushrooms; cook 6 to 8 minutes. Season with kosher salt and pepper. Add 1 1/4 cups chicken broth, 1 cup short-grain brown rice and 1/2 teaspoon salt. Put on and lock the lid. Set to pressure-cook on high for 25 minutes. Let the pressure release naturally. Stir in 2 tablespoons chopped parsley.