Heat 2 tbsp of olive oil in a large dutch oven over medium heat. Toss in onions, cellery and carrot, and sautee until light brown, about 10 minutes.
Add 2-3 cloves of minced garlic, 1 3 oz can of tomato paste and 15 thyme stems to the pot, and cook for about 60 seconds, stirring frequently. Pour in 4 cups of seafood stock and 1/2 a cup of water. Season with 1/2 tbsp of salt and 1/2 tsp pepper (use less salt if seafood stock is salty). Bring to a boil, reduce heat and simmer for about 1 hour, stirring occasionally. Once done simmering, strain the liquid to remove all of the solids (squeeze the solids to get all of the flavor out of them). You should have about 4 cups of liquid.
In a seperate pot, bring a few quarts of water to a boil. Once at a roaring boil, add the lobster tails and cook until fully cooked through, about 1 minute per ounce. Drain the water and let the lobsters cool a bit.
In another dutch oven or pot, heat 2 tbsp of olive oil. Cook the leeks until softened, about 10 minutes, then add 1 tbsp of minced garlic. Cook for 1 minute. Pour in the cognac and sherry, and simmer for 3 minutes over medium heat.
Once the lobsters are cool enough to handle, remove the lobster meat from the shells. Add 1 lobster tail, plus about 1/4 of the second tail, to the pot with the leeks. Using a hand blender, blend the ingredients until a puree forms. Remove the puree from dutch oven, and put aside.
In the same dutch oven as you cooked the leeks in, melt the butter. Add the flour slowly, whisking until combined. Add 1 tbsp of tomato paste to the pot, and whisk in the half and half slowly. Cook for about 3 minutes over medium heat. Add the pureed leek mixture back to this pot.
Chop up the rest of the lobster into bite sized pieces, and add it to the pot. Pour in your strained seafood stock, and mix well. Squeeze in 1 tbsp of lemon juice, and season with salt and pepper, garnish with parsley, dill or thyme and enjoy!