Slice off and discard the top of the head of garlic. Place the garlic in a microwave-safe bowl and drizzle with the oil; pour 2 tablespoons water into the bowl. Cover with a plate (to act as a lid) and microwave on 50-percent power until the garlic is very soft, 8 to 9 minutes. Let cool until comfortable to handle.
Meanwhile, slice the bread in half lengthwise.
Put the softened butter in a small bowl, squeeze in the garlic cloves and add the garlic salt. Stir until well blended. Generously spread the bread with garlic butter.
Preheat the broiler to high. Place the bread, buttered-side up, on a baking sheet and broil until lightly golden on top, 3 to 5 minutes. Serve warm.