California Burgers with Guacamole Mayonnaise

Ingredients:Mayonnaise2 tablespoons grated white onion1 ripe Hass avocado, pitted and peeled2 tablespoons mayonnaise2 plum tomatoes, cored, seeded, and chopped1 tablespoon finely chopped fresh cilantro leaves2 teaspoons fresh lime juice1 small garlic clove, gratedKosher salt2 poblano chile peppers1-1/2 pounds ground chuck (80% lean)1-1/2 teaspoons kosher salt1 teaspoon ground black pepper4 hamburger buns Directions: