Smoked Whitefish Chowder

1 1/2 pounds boneless smoked whitefish fillets
7 cups water
2 Tablespoons butter
1 small onion, diced
1 small fennel bulb, diced (save fronds for garnish)
2 celery sticks, diced
1/2 teaspoon dried thyme
1/2 teaspoon dry mustard
1 1/2 pounds Yukon Gold or redskin potatoes, diced
3 cooked ears of corn, kernels cut off the cob
OR 1 1/2 cups frozen corn
1 1/2 cups half and half

Place smoked fish fillets in a wide pot. Cover with 5 cups cold water. Place over medium high heat and bring to a simmer. Reduce heat to low and simmer for about 5 minutes. Transfer fish to plate and let cool. Reserve the poaching liquid. Discard skin and flake into chunks. Bring poaching liquid back to a simmer and reduce to about 3 cups.

Melt butter in a large, heavy pot over medium low heat. Add onion, fennel, celery, and thyme. Cook until soft. Stir in dry mustard. Set strainer over pot and pour in reserved poaching liquid. Add 2 cups of water and potatoes. Increase heat to medium high and simmer until potatoes are tender. Add salt and pepper to taste. Take potato masher and briefly smash down contents of soup to thicken. Turn off heat. Add corn, smoked fish, and half and half. Stir and garnish with fennel fronds. Serves 6 to 8.