In a large stockpot, heat butter over medium heat. Add onion, celery, carrots, and garlic and cook until softened.
Add flour to mixture and stir until the flour has been absorbed.
Add thyme and tomato paste and stir to combine.
Add the cognac and stir to combine.
Pour in the lobster/fish broth. Stir and bring to a boil.
Continue cooking until soup thickens.
Working in batches, add the soup to a blender and blend until smooth. Wipe the inside of the pot to remove any large chunks of vegetables that didn’t make it to the blender. You can opt to use a hand immersion blender to puree the soup.
When smooth, put the soup through a sieve and return to the soup pot.
Return to simmering and add the heavy cream, and stir until well combined. Taste and adjust as you are cooking.
To serve, pour soup into a bowl.
Garnish with the cooked, chopped lobster.
(Optional) Place some of the cooked lobster meat in the pot, leaving some for garnish on the soup when serving.
