Preheat oven to 350°F. Prepare your baking sheet by covering it with a generous piece of aluminum foil.
In a bowl combine the butter, garlic and salt. Stir to combine.
Cut vertical and horizontal cuts (using a serrated knife works best) in the round loaf, cutting just to the bottom of the crust (but not through the crust).
Use a butter knife and run herbed butter mixture through each slice, both horizontally and vertically, making sure to coat each piece. Insert cheese shreds throughout the bread slices as well, leaving some around the top for a nice, melted cheese look when your loaf is done. Sprinkle the top with onion slices.
Place the loaf on your prepared baking sheet. Tear off another piece of foil and cover the bread, sealing it to the piece of foil on the pan. Place the pan in the oven and bake for about 15 minutes.
Take the loaf out of the oven and carefully remove the top piece of foil. Return the pan to the oven and bake for until 10 minutes or so, until the bread has toasted edges and the cheese has melted.