Rinse the onion in a fine-mesh strainer under cold water and let the excess water drain off. In a medium bowl mash the avocado and mayonnaise together with a fork. Stir in the onion, tomatoes, cilantro, lime juice, and garlic. Season generously with salt. Cover with plastic wrap, pressing the wrap directly onto the surface, and set aside. (The mayonnaise can be prepared up to 8 hours ahead.)
Related Grill Skills
Dicing an Avocado
Mincing Garlic
Prepare the grill for direct cooking over high heat (230° to 290°C).
Prepare the grill for cooking over high heat (260°C).
Brush the cooking grates clean. Grill the poblano chiles over direct high heat, with the lid closed, until the skin is blackened on all sides, about 10 minutes, turning occasionally. Transfer the chiles to a bowl and cover with plastic wrap to trap the steam. Let stand for about 10 minutes. Remove the chiles from the bowl and peel off and discard the blackened skin, stems, and seeds. Chop the chiles into 12-millimeter dice.
Brush the cooking grates clean. Grill the poblano chiles over high heat, with the lid closed, until the skin is blackened on all sides, about 10 minutes, turning occasionally. Transfer the chiles to a bowl and cover with plastic wrap to trap the steam. Let stand for about 10 minutes. Remove the chiles from the bowl and peel off and discard the blackened skin, stems, and seeds. Chop the chiles into 12-millimeter dice.
Mix the ground chuck, chiles, 1½ teaspoons salt, and the pepper, and then shape into four patties of equal size, each about 18 millimeters thick. With your thumb or the back of a spoon, make a shallow indentation about 2.5 centimeters) wide in the center of the patties to keep them from forming a dome as they cook.
Grill the patties over direct high heat, with the lid closed, until cooked to medium doneness, 8 to 10 minutes, turning once when the patties release easily from the grate without sticking. During the last minute of cooking time, toast the buns, cut side down, over direct heat. Build each burger on a bun with a patty and a dollop of the guacamole mayonnaise. Serve warm.
Grill the patties over high heat, with the lid closed, until cooked to medium doneness, 8 to 10 minutes, turning once when the patties release easily from the grate without sticking. During the last minute of cooking time, toast the buns, cut side down, over direct heat. Build each burger on a bun with a patty and a dollop of the guacamole mayonnaise. Serve warm.
