In a large soup pot or skillet, heat the butter with the vegetable oil over medium heat. Stir in the onions, garlic, and leeks. Cook over low heat, covered, for approximately 10 to 15 minutes, stirring occasionally, until the onions are very tender.
Meanwhile, slice the corn kernels from the cobs.
Cutting corn off the cob: In a shallow bowl, hold ears of corn upright and, with a sharp knife, cut kernels from the cobs. Then with blunt edge of the knife, scrape juice from cobs. Set the corn aside and reserve the cobs for the broth.
Pour the 4 cups of water into the onion mixture; add the bay leaf and the corn cobs; bring the mixture just to a boil. Reduce the heat to a simmer and cook for about 20 minutes. Remove and discard the cobs.
Add the corn kernels and the heavy cream, and simmer until the mixture is reduced by 1/3 (about 35 minutes). Working in batches, puree the soup in a blender or food processor; then pass the soup through a fine sieve. Reheat the soup over medium heat, stirring occasionally, until hot. Season with salt, white pepper, and Tabasco.
To serve, ladle into pre-warmed soup bowls. Distribute the crab evenly in the center of the soup bowls and garnish each bowl with chopped chives. Dot with red chili oil around the edges.
Makes 6 servings.