Remove and discard tough ends from broccoli rabe stems; coarsley chop the broccoli rabe.
Heat oil in a large Dutch oven over medium heat. Add onion; cook 5 minutes, stirring frequently. Add red pepper and 2 garlic cloves; cook 1 minute, stirring constantly. Add broccoli rabe and salt, and cook 30 seconds, stirring constantly. Add water and cheese rind. Increase heat to medium-high; bring to a boil. Reduce heat, simmer 15 minutes. Add beans, and simmer 5 minutes. Discard cheese rind.
Rub both sides of toasted bread with cut sides of halved garlic clove. Tear toasts into bite-sized pieces; divide toast pieces evenly among 4 bowls. Ladle 1 1/2 cups soup over toast pieces in each bowl; top each serving with 2 tablespoons grated cheese.