Place a large heavy bottomed pot over medium heat. Add olive oil and shallots, stirring frequently until the shallots are softened- about three minutes. Next add the tomatoes, and sherry (be careful when adding alcohol with an open flame) and simmer while stirring for about two minutes. Add fish stock and lemon juice to the pot. Turn down heat to maintain a gentle simmer, and cook for fifteen minutes, allowing the flavors to develop. Reduce the heat to low, and add in the initial quantities of cream, thyme, salt, and cayenne pepper. Stir to combine, and then taste- adjusting the ingredients as necessary until it is flavored to your liking. Once the broth is finished add in the lump crab. Stir the soup and cook for five minutes, or just until the cream and crab are warmed through. Turn off the heat and serve immediately.