Bring a medium saucepan of water to a boil, and then add 1/2 tablespoon of salt. Cut the thawed lobster tails down the top. Start opposite the fins and cut straight down, stopping 1 inch from the end to leave the fins intact.
Add the tails to the pot and boil for 1 minute per ounce. Remove from the pot and set aside to cool. Then, remove the meat from the shells and chop it into small pieces for later.
In a large pot, melt 4 tablespoons of butter over medium heat. Add the onion, celery, carrots, ½ teaspoon of salt, dried thyme, and black pepper and cook for 5-7 minutes, stirring occasionally.
Stir in tomato paste and all-purpose flour and cook for about 2 minutes. Pour in the white wine and sherry, stir, and cook until the liquid is reduced by about half.
Add the seafood stock, stir, and lower the heat to a low boil for 30 minutes. Stir occasionally.
Remove the pot from heat and allow it to cool slightly. Use an immersion blender to puree the soup until smooth. (You can also do this in a blender. Work in batches and then add blended soup back to the pot.) Bring the soup back to a low boil before stirring in the heavy cream.
While the soup is heating, place a medium skillet over medium heat and melt the remaining 2 tablespoons of butter. Add the garlic and cook for 1 minute, stirring constantly. Add the chopped lobster meat and 1/2 teaspoon of salt. Turn the heat to low and cook for 1-2 minutes to infuse the flavors.
If garnishing the soup with lobster meat, set aside about ¼ of the meat. Add the rest of the lobster meat to the broth and stir to combine.
Portion the soup into bowls and garnish with chives and the reserved lobster meat.
Notes
If you can't find seafood stock at the store, it's really easy to make.
Seafood Stock
Ingredients
1 large yellow onion, halved
2 stalks celery, halved
1 carrot, halved
3 cloves garlic, smashed
5 peppercorns (or 1/8 teaspoon black pepper)
4 sprigs fresh thyme
2 bay leaves
Lobster shells (use the two shells after you cook the lobster meat)
Water
Directions
Add everything except the water to a large pot and press down to fill any gaps between the ingredients.
Pour in enough water to cover (at least 4 1/2 cups), and then bring to a boil. Lower the heat to a simmer and cook for 20-30 minutes.
Strain the stock through a fine mesh sieve. Stor in an airtight container in the fridge for up to 4 days or freeze for up to 6 months.
The lobster meat and stock can be made ahead of time. Just store the lobster meat in an airtight container in the fridge. You'll warm it up in the garlic butter before adding it to the soup.