Preheat oven to 425°F. Line a large rimmed baking sheet with foil or parchment.
Trim the sprouts and halve any larger ones so all pieces are similar in size.
Whisk olive oil, Dijon, honey, vinegar, garlic, salt, pepper, and red pepper flakes in a large bowl.
Add Brussels sprouts and toss with about two-thirds of the honey-Dijon mixture until well coated.
Spread sprouts cut-sides down on the sheet. Roast 15 minutes, then stir and flip.
Roast 8–12 minutes more until deeply browned and tender. Toss with remaining mixture. Broil 1–2 minutes if you want extra caramelization.
Finish with parsley and serve hot.