Preparation 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.
Preparation 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.