Step 1

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, 1/2 teaspoon salt and ¼ teaspoon pepper. Measure ¼ cup of the mixture into a small bowl and set aside.

Step 2

Place the cauliflower on the prepared baking sheet and brush the surface with the remaining oil-mustard mixture. Roast until deeply browned and a skewer inserted all the way through the head and into the core meets just a little resistance, 40 to 55 minutes.

Step 3

While the cauliflower roasts, in a small bowl, toss together the feta and parsley. When the cauliflower is ready, remove the baking sheet from the oven. Brush the remaining 1½ tablespoons mustard over the surface, then pat on the feta-parsley mixture, pressing so that it adheres. Return to the oven and continue to roast until the feta begins to melt, 5 to 8 minutes.

Step 4

Using a wide metal spatula, transfer the cauliflower to a cutting board, then cut the head into 4 wedges. Serve with lemon wedges and the reserved oil-mustard mixture.