Prep: Preheat oven to 400°F. Trim outer leaves from cauliflower and cut stem flush with base so head sits upright.
Fit Test: Place cauliflower in 4½-quart Dutch oven. If lid doesn't fit, trim stem until lid closes properly.
Make Seasoning: In a glass measuring cup, whisk together olive oil, Dijon mustard, apple cider vinegar, oregano, chives, garlic powder, onion powder, salt, pepper, and red pepper flakes until well combined.
Season: Pour mixture over cauliflower. Using clean fingers, rub seasoning into all surfaces and crevices until completely coated.
First Roast: Cover with lid and roast 35 minutes, or until knife inserted in thickest part meets no resistance.
Broil: Remove lid, switch oven to broil, and broil 5 minutes until golden brown and crispy on top.
Rest and Serve: Carefully transfer to serving plate. Let rest 5 minutes, then slice into wedges. Garnish with fresh parsley and drizzle with olive oil if desired.