Position an oven rack at the bottom of the oven and pre-heat to 450 degrees F. Line a baking sheet with parchment paper or aluminum foil. Remove the leaves from the cauliflower and trim the stems, so the cauliflower heads sit flat on the prepared baking sheet. Rub the outside of each head with the garlic cloves. In a small bowl, whisk together the olive oil, 3 tablespoons of the mustard, a pinch of salt and a few grinds of black pepper.
Place the cauliflower heads on the prepared baking sheet and brush the entire outside and inside with the mustard-oil mixture. Roast the cauliflower for 50 to 60 minutes, or until it's nicely charred and tender inside. A long skewer or knife inserted through the center, should pass through easily. Remove the tray from the oven and let the cauliflower cool for a few minutes.
Brush the remaining 1 tablespoon of mustard on the cauliflower heads. Cut the cauliflower into thick wedges and serve on plates with a sprinkle of sea salt and a squeeze of fresh lemon juice.