Melt 8 tablespoons butter in a 9-inch skillet over medium heat. Add the onion and cook for 5 minutes until soft but not browned. Stir in the tarragon and parsley. If using raw shrimp or scallops, add them with the herbs and cook for 1 minute. Remove from the heat and let cool slightly. If using cooked lobster or crabmeat, remove the pan from the heat as soon as you stir in the herbs, let cool, and then add the lobster or crabmeat. Season with salt and pepper.
With a cleaver of chef's knife, split the lobsters in half lengthwise. Remove and discard the head sac and intestine. Remove the tomalley and the roe if present and place in a small bowl. Break into small pieces using a fork. With the back side of a knife, crack the center of each claw on 1 side only. Season the lobsters lightly with salt and pepper. On a large roasting pan or baking sheet, place the halves together to resemble a butterfly.
The tomalley and roe are optional for the stuffing. If you want to include them, mix them into the seafood mixture. Gently fold the crumbled crackers into the mixture. Divide the mixture evenly between 2 lobsters. If you are serving 1 lobster per person, spread the stuffing over the center so that the lobsters look whole again. Do not pack the stuffing tightly, or it will affect the even baking of the lobster. Brush the 3 tablespoons melted butter over the exposed meat tail, stuffing and claws. Bake until the lobster is cooked through and the stuffing is crisp and golden. Bake 17 minutes for a 1 1/2 pound lobster and 24 minutes for a 2-pound lobster.