Before baking, preheat oven to 325°F / 160°C /Gas Mark 3
Place bagels flat on cutting board. With a serrated knife, cut bagels in half vertically. Place halves on cutting board, cut side down. Cut halves into thin slices, about 1/4 inch thick. Repeat with remaining bagels.
Place slices on ungreased baking sheets. In a small bowl, blend butter with garlic salt and onion salt. Spread over tops of bagel slices and bake 15-20 minutes or until lightly browned. Cool on a wire rack.