In a large heavy skillet, preferably cast iron, heat the olive oil over medium-low heat. Add the garlic and cook very slowly, stirring frequently, until the garlic has turned golden, about 3 or 4 minutes. Be very careful not to let the garlic scorch or burn. Transfer the mixture to a shallow ceramic baking dish and let cool. Add the prawns and the chile to the garlicky oil, toss to coat them evenly, and refrigerate overnight, covered. Make the sauces below and, when ready to serve, preheat a grill or broiler to medium heat. Grill the prawns for 2 to 3 minutes on each side, then transfer to a warm platter. Scatter the salt and lime juice over them and serve immediately, accompanied by the Avocado Mayonnaise and the Ancho Chile Mayonnaise.