Place squash cubes and garlic in slow cooker with 1/4 cup water. Cover and cook on HIGH for 3 hours or on LOW for 5 hours or until squash is tender.
Meanwhile, heat oil in a medium-size nonstick skillet over medium-high heat. Cook onion in skillet for 5 minutes or until softened. Stir ginger into skillet and cook 1 minute.
Heat oven to 350 degrees. Remove squash from slow cooker using a slotted spoon and discard garlic and remaining liquid in slow cooker. Puree half of squash in blender or food processor with half of onion mixture and 2 cups of the broth until completely smooth; pour into slow cooker and repeat with remaining squash, onion mixture, 1 cup broth and 3/4 cup water. Stir in salt and pepper, then cover soup and cook an additional 30 minutes or until hot.
While soup is heating up, if desired, place bread slices on a baking sheet and top each with 1 teaspoon cheese. Bake at 350 degrees for 15 minutes or until lightly browned. Place 1 bread slice on top of soup in each serving bowl.