Line a baking sheet with aluminum foil.

2)Mix olive oil, Dijon mustard, sea salt, and red pepper flakes in a large bowl to make sauce.

3)Trim the stems of the Brussels sprouts to release outermost leaves, about 5 leaves from each sprout.

Reserve cores to roast another time.

Place leaves in the bowl of sauce; toss with your fingers until all the leaves are evenly coated.

Spread leaves on the baking sheet in a single layer.

4)Roast leaves in the preheated oven until wilted and lightly browned, about 15 minutes.

Transfer any crispy leaves to a serving plate and continue roasting, removing crispy leaves at 5-minute intervals, until all leaves are crispy, about 10 minutes more.