Cut bread diagonally into 1-in. slices to within 1 in. of bottom. Repeat cuts in opposite direction. Cut Swiss cheese into 1/4-in. slices, cut slices into small pieces. Insert into bread. Combine the onion, butter, garlic and salt, spoon over bread.
Wrap loaf in foil. Bake at 350 for 25-30 minutes or until cheese is melted. Serve warm.