If your slow cooker has a stovetop/sear function, you can do everything in one pot. Otherwise, begin by using a skillet for steps 1-3 before transferring to the slow cooker for step 4.

Start by heating the olive oil and butter in the slow cooker set to the sear mode (or in a skillet over medium heat). Once the butter has melted, add the leeks, garlic, red pepper flakes, and a small pinch of salt. Cook for about 5 minutes until the leeks soften.

Mix in the tomato paste and cook for another minute, allowing it to blend with the leeks. Pour in the sherry or Marsala wine and let it reduce by half, which should take a few minutes. If you're using a skillet, transfer the leek mixture into the slow cooker at this point, making sure to include any liquids.

Add the seafood broth, diced tomatoes, Old Bay seasoning, and sugar into the slow cooker. Stir everything together until well combined.

In a separate bowl or cup, whisk the flour with the water until smooth, then stir it into the soup base. Add the kosher salt and black pepper, stirring to combine.

Cover the slow cooker and cook on HIGH for 2 hours (or on LOW for 4 hours).

After the cooking time, taste the bisque and adjust the seasoning if necessary. Stir in the shrimp, crabmeat, cream, and chopped parsley. Cook for an additional 15 minutes until the shrimp are pink and fully cooked. Keep the bisque warm until you're ready to serve.

For the crab topping, mix together the lemon juice, mayonnaise, Dijon mustard, whole grain mustard, and parsley in a bowl. Gently fold in the crabmeat, taking care not to break it up too much. Season with salt to taste. Keep the topping chilled until ready to serve.