Plancha Style Shrimp

Name: Plancha-Style Shrimp
Ingredients: 1/4 cup vegetable oil; 4 garlic cloves; 24 large shrimp (about 1 1/2 lb.), middle sections peeled; heads and tails left intact, if desired; 1 medium red onion, thinly sliced; Kosher salt and freshly ground black pepper; 2 tablespoons chopped flat-leaf parsley; Lemon wedges