Preheat an oven to 400 degrees. Line a the bottom of a Dutch oven with parchment paper.
Next, remove the leaves and stem from the cauliflower so it sits flat. Place it on a baking sheet.
In a blender or food processor combine garlic, chili powder, paprika, chipotle peppers, pineapple juice, vinegar, pineapple chunks, salt and pepper. Blend the mixture until smooth.
Using a pastry brush take the al pastor sauce and brush it all over the cauliflower. Reserve at least at least 1/3 cup for basting later.
Place the cauliflower on top of the parchment paper in the Dutch oven. Pour 1/4 cup of water into the bottom of the Dutch oven and cover with a lid. Roast the cauliflower covered for about 45 minutes.
Remove the lid and brush the cauliflower with remaining sauce. Continue to cook the cauliflower uncovered for another 45 minutes to an hour. Once cooked, remove from oven and allow to cool for a few minutes. Serve with garnishes.