Melt the butter in a large soup pot over a medium-high setting. Add the leeks, ginger and garlic and saute for 2 minutes.
Add the celery, thyme, and salt and pepper to taste. Continue to cook until tender, about another 3 to 4 minutes.
Add the potatoes, carrots, lemon zest and cayenne to the pot, and cook, stirring constantly, for another 2 minutes.
Sprinkle the flour over the mixture and stir it into the vegetables for 2 or 3 minutes.
Stir in the fish or chicken stock, making sure to stir in any bits of flour on the bottom. Add the milk and cream, and increase the setting to high. Stir as you bring the pot to a boil.
Add the fish and shrimp and stir into the soup. Reduce the heat to medium-low or low, and let the chowder simmer for another 20 to 30 minutes, stirring occasionally.