Preheat oven to 450°F. Place cauliflower on a parchment-lined rimmed baking sheet and brush with ¼ cup oil; season with salt and pepper. .
Pull short sides of parchment toward middle to meet one another; fold or roll downward a few times until you reach cauliflower. Working with one side at a time, fold open ends inward (like wrapping a present) and tuck underneath cauliflower.
Roast until knife-tender, about 40 minutes. Tear open parchment at top; roast until golden brown, 15 to 20 minutes more.
Meanwhile, stir together parsley, cilantro, garlic, mustard, vinegar, and remaining ½ cup oil in a small bowl to combine. Season with salt and pepper. Serve cauliflower warm, with sauce alongside.
