Preheat oven to 450°. Line a large baking sheet with parchment paper.
Cut cabbage in half, then cut each half into four equal wedges. Leave core intact so leaves stay together while roasting.
Arrange on baking sheet, drizzle with olive oil, and sprinkle with salt and pepper.
Bake 10–20 minutes, flip wedges, and roast until browned and tender.
For the sauce, add butter, onion, mustard, and garlic to a small saucepan and cook over medium heat until butter is melted.
Let sauce cool slightly. Pour into a lidded container—a glass jar works well—and shake well before pouring over cabbage wedges.
Garnish with chives or parsley, if desired.