Preheat oven to 350°. Cut bread diagonally into 1-in. slices to within 1 in. of bottom of loaf. Repeat cuts in opposite direction. Place cheese cubes and mushrooms in cuts. , In a microwave-safe bowl, combine remaining ingredients; microwave, covered, on high until butter is melted, 30-60 seconds. Stir until blended. Spoon over bread. , Wrap loaf in foil; place on a baking sheet. Bake until cheese is melted, about 40 minutes.