- 12 broccolini stalks, cut to 4-inch length, or 12 broccoli florets
- 4 large (3×5-inch) slices rye bread
- 3 tablespoons mustard-flavored mayonnaise (such as Dijonnaise)
- 4 thin red onion slices
- 1 cup (packed) shredded sharp cheddar cheese
- Preheat broiler. Steam broccolini until just tender, about 5 minutes. Transfer to paper towels and pat dry. Broil bread on baking sheet until lightly toasted, about 1 minute per side. Spread top side of each bread slice with mustard-flavored mayonnaise. Top each with onion slice, broccolini and cheese. Broil until cheese melts and browns, about 2 minutes.