Note: for best flavor, make the filling ahead and stuff the shrimp 4 to 6 hours before cooking.
Heat 1 tablespoon of the olive oil in a small skillet over medium heat. Add the bread crumbs and garlic and reduce the heat to low. Cook, stirring, until the bread crumbs are golden, about 3 minutes. Transfer to a small mixing bowl and add the parsley and onion. Toss to combine well and season with salt and pepper, to taste.
Using a sharp paring knife, make a long slit along the backside of each shrimp, taking care not to cut through to the other side. Divide the filling evenly among the shrimp, pressing the stuffing into each pocket. Using a pastry brush, lightly coat each shrimp on both sides with some of the remaining olive oil and season lightly with salt and pepper.
Preheat the broiler to high and place the oven rack as close to the flame as possible. Place the shrimp on a baking dish and broil until bright pink on 1 side, about 2 minutes. Turn shrimp over and continue to cook until just cooked through, 1 to 3 minutes longer. Serve immediately with lemon wedges on the side.
Yield: 4 appetizer servings