Heat oven to 375°F. In 15x10x1-inch pan or cookie sheet with sides, melt 3 tablespoons butter in oven. Add bread cubes and garlic powder to butter; toss well to combine. Bake about 10 minutes, stirring occasionally, until bread cubes are crisp and brown.
Meanwhile, in 2-quart saucepan, melt 3 tablespoons butter over medium heat. Add white onion; cook over medium heat 5 to 7 minutes, stirring occasionally, until onion is translucent. Stir in water, soup base, green onions and pepper. Heat to boiling; reduce heat. Simmer uncovered 10 minutes. Stir before serving.
Top each serving with 1/4 cup croutons and 1 tablespoon cheese.