Clean the charr and remove the head and tail. If the fish is large, cut it into 2 or 3 pieces.
In a large saucepan or Dutch oven, cook the diced salt pork over medium heat until crisp. Remove the pork with a slotted spoon and set aside, leaving the drippings in the pan.
Add the butter to the pan and melt. Add the chopped onion, celery, and carrots and cook until softened, about 5-7 minutes.
Add the white wine and scrape up any browned bits from the bottom of the pan. Let the wine simmer for 2 minutes.
Add the fish stock (or clam juice), bay leaf, and thyme. Bring to a simmer.
Season the charr pieces with salt and pepper. Place the charr in the simmering liquid. The liquid should come about halfway up the sides of the fish.
Cover the pan and simmer gently for 10-15 minutes, or until the fish is cooked through and flakes easily with a fork. Cooking time will depend on the thickness of the fish.
Carefully remove the charr from the pan and place on a warm serving platter.
Increase the heat under the pan and boil the sauce until it has reduced slightly, about 5 minutes. Strain the sauce, discarding the solids.
Stir the reserved crisp salt pork, chopped parsley, and lemon juice into the strained sauce.