Using a serrated knife, cut bread diagonally into 1-inch thick slices to within 1 inch of the bottom of the loaf. Repeat in the opposite direction.
Cut the Swiss cheese into 1/2 inch cubes. Stuff cheese cubes and mushroom slices into the slits in the bread.
Combine the butter, poppy seeds, minced garlic, seasoned salt, ground mustard, and lemon juice in a microwavable dish. Cover and microwave on high for about 2 minutes, until butter is melted. Stir to combine well.
Spoon butter mixture over bread.
Wrap bread in foil and bake at 350 degrees for about 40 minutes.