Coat a Dutch oven with cooking spray; place over medium-high heat until hot. Add onion; saute 5 minutes or until tender. Add water; reduce heat to medium-low. Cook, uncovered, 25 to 30 minutes or until onion is golden, stirring occasionally.
Sprinkle flour over onion; gradually stir in 1 can broth. Bring to a boil, stirring constantly. Add remaining 2 cans broth, potato, and next 5 ingredients. Bring to a boil; cover, reduce heat, and simmer 15 minutes.
Place 6 ovenproof bowls on a baking sheet. Place 1 bread slice in each bowl; ladle 1 1/2 cups soup into each bowl. Sprinkle evenly with cheese. Broil 5 1/2 inches from heat (with electric oven door partially opened) 4 minutes or until cheese is golden.