Heat the oven to 450°F with a rack in the middle position. Line a rimmed baking sheet with foil or kitchen parchment.
In a small bowl, whisk until creamy the oil, 2½ tablespoons of mustard, the vinegar, honey, garlic, ½ teaspoon salt and ¼ teaspoon pepper. Measure ¼ cup of the mixture into a small bowl and set aside.
Remove the leaves from the cauliflower, if desired, but keep the core intact. Place the cauliflower on the prepared baking sheet and brush all over with the remaining mustard-oil mixture
Roast for about 30 minutes, or until the cauliflower is browned and tender when pierced with a skewer
Brush the reserved mustard-oil mixture over the cauliflower
Sprinkle the feta and parsley over the cauliflower, then roast for an additional 5 minutes, or until the feta is softened
Remove the cauliflower from the oven and let cool for 10 minutes. Cut the cauliflower into wedges and serve