Melt the butter in a pot set over medium heat.Add the onion, celery, and garlic and cook until softened, about 5 minutes. Mix in the flour and cook 2 minutes more. Slowly, stirring constantly, pour in 1 cup of the stock. When the mixture has thickened, mix in the remaining stock.
Stir in tarragon, corn, potatoes, and dulse and bring to a simmer. Simmer until the potatoes are tender and the dulse has softened, about 12 to 15 minutes. Mix in the crab, reserving a bit to garnish the top of the chowder, if desired. Simmer the chowder 2 minutes more.
Taste the chowder and season with white pepper, to taste. (The dulse and crab will add a salty taste to the chowder.) Ladle chowder into bowls and garnish, if desired, with a little crab meat.
Note: When you cut the kernels off 1 large cob of corn, you should get the 1 cup required for this recipe. Dulse, which is sold in small bags, can be snipped into small pieces for this recipe with kitchen scissors; dulse and fresh crab meat
are sold in our Seafood Department.