Preheat the oven to 450°F. Line a baking sheet with parchment paper.
Remove the leaves from the cauliflower. Cut the stem so it is level with the bottom of the head and is able to sit upright on the baking sheet.
Turn the cauliflower upside down and gently stuff the garlic slivers between the florets all around the head.
In a small bowl, whisk together the olive oil, honey, vinegar and half of the Dijon mustard. Add salt and black pepper to taste.
Sit the cauliflower upright on the baking sheet and brush the entire surface with the olive oil mixture. Roast the cauliflower until dark golden brown and tender, about 35-40 minutes depending on the size of your cauliflower.
Combine parsley and feta in a small bowl. After the cauliflower has cooked through, remove the head and brush the remaining 2 Tbsp of mustard on the surface. Rub with feta and parsley mixture and roast for 5 more minutes until feta melts.