Melt 2 tbsp butter in a medium skillet over medium-high heat. Add the green pepper, celery, and onion, stir to coat, and sauté until the veggies have softened, 5-8 minutes.
Add the chopped shrimp to the skillet and stir into the veggies. Cook until the shrimp have turned pink.
Stir the chicken broth, Worcestershire sauce, and dried dill into the skillet. Let the mixture heat through, then turn off the stove and remove the skillet from the heat.
Stir the bread crumbs into the skillet until you have a thick stuffing mixture. Taste and additionally season with salt and pepper to taste. Set stuffing aside to cool slightly.
Liberally season both sides of flounder filets with salt, pepper, onion powder, and a dash of paprika. Place seasoned filets in a single layer in the prepared baking dish and squeeze the juice of half a lemon over the fish.
Spoon the shrimp stuffing on top of the flounder, covering as much of the filets as possible.
Slice the remaining 4 tbsp of butter into thin pats and place on top of the stuffing all around the baking dish.
Liberally season the flounder and stuffing with salt, pepper, onion powder, and paprika.
Bake the flounder at 375F for 10-15 minutes, or until fish flakes easily with a fork. Serve Baked Flounder with Shrimp Stuffing while still hot from the oven.