Instructions

1. Prepare and preheat grill.
2. In a small bowl, combine chili powder, cumin, garlic powder, onion powder, and 1 1/2 teaspoons salt to make spice mixture, stirring to combine thoroughly.
3. Peel the onion and cut away the stem and root ends, then cut the onion in 4 even pieces horizontally, keeping the concentric rings nestled together in planks. Remove the stem and core from each pepper and cut each into four large pieces (cut down through each vertically and then in half horizontally). Toss or brush the vegetables with olive oil, making sure they are coated thoroughly (and keeping the onion planks together in concentric rings). Sprinkle vegetables with 2 tablespoons of the spice mixture, evenly coating both sides of all vegetable pieces.
4. In a large bowl, crumble ground beef and sprinkle evenly with remaining 5 teaspoons of spice mixture, mixing gently but thoroughly to be sure spices are evenly distributed throughout the meat. Form 4 burger patties (each about 1/2 - 3/4" thick and 3 1/2 - 4" in diameter), pressing gently but firmly (overworking the meat can lead to tough burgers), and pressing to make a small divot in the center of each (see note).
5. Grill burgers and vegetables over medium heat for about 8-10 minutes on the first side and 5-8 minutes more on the second side, until vegetables are crisp-tender and burgers have reached your desired level of doneness. If your onion is fairly large and yields thick planks, you may want to put those over the hottest section or grill them for an extra minute or two. Also, poblanos are generally thinner-fleshed than red peppers, so they will do better if placed over a cooler section, or may need to be pulled off the grill a minute or two early. (Remember that grills vary widely in temperature settings, so the settings and times you use may need to be adjusted according to your specific grill.)
6. Meanwhile, in a medium bowl, combine avocado, salsa, 1/8 teaspoon salt, and black pepper, mashing and mixing to make a chunky guacamole.
7. Remove finished burgers and vegetables from the grill, allowing burgers to rest for a few minutes. Meanwhile, slice vegetables into strips, cutting each pepper piece into 4-6 pieces, and cutting each onion plank in half and separating rings.
8. Serve each burger on a whole grain bun, topped with lots of vegetable strips and a generous dollop of guacamole (and with knives, forks and plenty of napkins).