Preheat the oven to 425 degrees Fahrenheit. Line a large, rimmed baking sheet with parchment paper.
On the baking sheet, toss the cauliflower and garlic cloves with 2 tablespoons of the olive oil until lightly and evenly coated. Arrange the cauliflower in a single layer and sprinkle lightly with salt. Bake until the cauliflower is tender and caramelized on the edges, 25 to 35 minutes, tossing halfway.
When the cauliflower is almost done, in a Dutch oven or soup pot, warm the remaining 1 tablespoon olive oil over medium heat until shimmering. Add the onion, celery and carrots and ¼ teaspoon salt. Cook, stirring occasionally, until the vegetables have softened and the onion is turning translucent, 7-9 minutes. Then, add the water or broth.
Remove the skins from the garlic cloves.
Reserve a few of the prettiest roasted cauliflower florets for garnish. Then transfer the garlic cloves and the rest of the roasted cauliflower to the pot.
Add the cumin and nutmeg to the pot as well. Then, increase the heat to medium-high to bring the mixture to a boil. When the mixture has reached a boil, reduce the heat to maintain a gentle simmer, and cover. Cook, stirring occasionally, for 20 minutes.
Remove from the heat and let cool for a few minutes. Remove the sprigs of thyme from the pot.
Add the optional coconut milk or macadamia nuts if you plan to use them in the dish. Then, carefully transfer the soup to a blender, working in batches if necessary. Puree until smooth. Add the lemon juice and blend again until it's integrated into the soup. (Alternatively, use an immersion blender to puree the mixture in the pot). Add additional salt to taste. You can also add a little more lemon juice if it needs the extra zing.
Top each bowl. of soup with the reserved roasted cauliflower florets, sprinkles of fresh thyme (and red pepper flakes if you like some heat).