Place cereal in resealable food-storage plastic bag; seal bag and crush with rolling pin or meat mallet (or crush in food processor).
In medium bowl, mash beans with fork until no whole beans remain. Add crushed cereal, oats, onions and egg; mix well. Shape mixture into 5 patties, each about 3 1/2 inches in diameter.
Spray 12-inch skillet with cooking spray. Add patties; cook over medium heat about 10 minutes, turning once, until brown.
For each sandwich, place 1/4 cup spinach leaves and 1 tomato slice on bottom half of bun; top with bean burger and top half of bun.