How to Make It
Roast the habanero over a gas flame until lightly charred all over. Using a paper towel, rub off as much skin as possible. Remove the stem and seeds and transfer the chile to a blender. Add the watermelon and garlic and puree; transfer to a bowl and refrigerate.
Bring a large saucepan of water to a boil and fill a bowl with ice water. Score the tomato bottoms with an X. Add the tomatoes to the boiling water and blanch for 10 seconds to loosen their skins. Transfer the tomatoes to the ice water to cool.
Peel the tomatoes and halve them crosswise. Working over a sieve set in a bowl, pry out the seeds. Press on the seeds to extract the juice, then discard them. Transfer the tomatoes and their juices to the blender and puree. Stir the tomato puree into the watermelon puree along with the vinegar and season the gazpacho with salt.
In a medium bowl, whisk the lemon juice with the buttermilk and olive oil and season with salt. Fold in the crab, avocado and poblano. Ladle the soup into bowls, mound the crab salad in the center and serve.