Slow Cooker Shrimp and Crab Chowder

Name: Slow Cooker Shrimp and Crab Chowder
Ingredients: 1 tablespoon olive oil; 2 tablespoons unsalted butter; 3 leeks, washed and sliced; 4 garlic cloves, peeled and minced; red pepper flakes; 2 tablespoons tomato paste; 4 cups (1 L) fish stock; 2 cups (500 ml) water; 15 oz (450 ml) can of diced tomatoes; 1 teaspoon Old Bay Seasoning; 1 tablespoon sugar; 1 cup (250 ml) dry sherry; 2 teaspoons kosher salt; 1 teaspoon freshly ground black pepper; 1/4 cup all-purpose flour; 1 cup (250 ml) heavy cream; 1 pound (480g) peeled and deveined shrimp; 1 cup fresh jumbo lamb crabs; 2 tablespoons chopped fresh parsley