Crack the shell of the cooked lobster using a rolling pin or meat mallet and set the shells aside. Remove the lobster meat, chop it, and refrigerate it together with the crabmeat.

Heat olive oil in a deep skillet and add the lobster shells with salt and black pepper. Cook briefly until fragrant, then add celery and red onion and cook until softened.

Pour in the vegetable stock and allow it to simmer so the shells release their flavor. Strain the broth and discard the shells.

Melt butter in the skillet and stir in apple cider vinegar, garlic, and shallot until aromatic. Add this mixture to the strained broth.

Cut a small section from the end of the lobster tails and add it to the broth, keeping the rest of the tails refrigerated.

Transfer the broth to the slow cooker and add diced tomatoes, cherry tomatoes, Old Bay seasoning, red pepper flakes, garlic powder, and onion powder. Cover and cook on low heat until deeply flavored.

Add the lobster tails near the end of cooking and allow them to turn red and gently cook through.

Remove all lobster tails, take the meat from the shells, and chop it. Combine with the chilled lobster and crab meat.

Blend the soup base until smooth using an immersion blender. Stir in heavy cream, chives, and all seafood meat.

Warm gently and serve hot with parsley, shaved Parmesan, and croutons.