Crush 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 with the crabmeat.

Heat a deep skillet over medium high heat, add olive oil, then add lobster shells, salt, and black pepper. Cook briefly until aromatic.

Add celery and red onion to the skillet and cook until slightly softened, then reduce heat and pour in the vegetable stock. Simmer to infuse flavor.

Strain the broth into a bowl and discard the shells. In the same skillet, melt butter, add apple cider vinegar, garlic, and shallot, then cook until fragrant. Add this mixture to the broth.

Cut a small portion from the lobster tails and add to the broth, reserving the rest of the tails in the refrigerator.

Pour the broth into the slow cooker and add diced tomatoes, cherry tomatoes, Old Bay seasoning, red pepper flakes, garlic powder, and onion powder. Cook on low until flavors are well developed.

Add the reserved lobster tails near the end of cooking and cook until just done. Remove tails, extract meat, chop, and discard shells.

Combine all lobster meat with crabmeat, season lightly, and set aside.

Blend the soup in the slow cooker until smooth, then gently stir in heavy cream, chives, lobster, and crabmeat. Heat through gently.

Serve hot, garnished with parsley, shaved Parmesan, and croutons.