On a gutted, gilled, scaled flounder, I slice along the backbone then fillet out in both directions to the fins, but don't cut thru at the fins. Leave the 2 flaps so that you can fold them back down into place. I leave the head on and only stuff the top side of the fish
Saute over medium heat - 2 stalks of celery finely diced, 3 cloves of garlic minced and about a teaspoon of minced yellow onion (EDIT: I have taken to using 2-3 green onions thinly sliced in lieu of minced yellow onion. A little less sweetness and a better flavor IMO) - in about 4 tbl. butter. Add the garlic last and don't burn it. Saute 5 minutes tops. Pour into a mixing bowl and let cool slightly.
Add Italian bread crumbs (and maybe a few crushed saltines), salt pepper and seasonings you prefer to taste (I use a little dried orregano, basil and Tony's), diced boiled shrimp, lump crabmeat (**a little lemon juice) and turn softly. Once mixed, add a beaten egg and mix again.
Stuff under the flaps and fold them back into place.
Bake at 375 degrees, basting with butter and (**a little lemon juice) a couple of times, until nearly done in the thickest part. Then baste again and put under the broiler until the edges are golden brown and crispy.
** I never add lemon until the fish is done, because I like the taste and zing of fresh lemon and am not too fond of the smell and taste of baked lemon, but that's just me. Some people prefer lemon juice in their suffing and/or on the fish while it cooks. It's a preference thing and I encourage you to go your own way.