Preheat the oven to 425°F.
Arrange the cauliflower florets on a baking sheet lined with parchment paper. Drizzle with olive oil, add salt and pepper, and sprinkle with the hot pepper flakes.
Roast for 20 minutes, then remove the baking sheet from oven and add the garlic clove to roast it until it melts. Return the baking sheet to the oven for 10 minutes or until the cauliflower is tender and begins to grill. Remove from the oven, and set aside the garlic clove to use in the sauce.
Place the avocado in a bowl. Press the garlic clove, remove the flesh and add it to the bowl. Add the rest of the ingredients. Using an electric or hand mixer, reduce the mixture to a smooth sauce. Adjust the consistency as needed by adding water or more lemon juice.
Arrange the cauliflower on a large serving plate accompanied by the avocado sauce.