Hold poblano pepper with tongs over gas flame of burner or broil until charred on all sides. (Or place pepper on a foil lined baking sheet. Bake in a 425 degree F oven for 20 to 25 minutes or until skin is bubbly and browned.) Wrap in foil and lets stand for 20 to 30 minutes or until cool enough to handle.
When cooled slip off blackened skin, twist off stem, and scoop out seeds and discard.
Coarsely chop and transfer to small bowl.
Add half of the avocado and coarsely mash with a fork.
Slice the remaining half of the avocado and add to the bowl.
Stir in lime juice, cilantro and mayonnaise. Season to taste with salt.
Refrigerate until time to assemble burgers.
In medium saucepan of boiling water, heat onion slices 30 seconds. Transfer to bowl of ice water to cool. Drain.
In second bowl combine cooled onions, lime juice, vinegar, cumin, salt, and pepper. Toss to coat. Let stand 30 minutes.
In large skillet over medium-high heat, cook chorizo about 5 minutes, until slightly browned and becoming crisp, breaking up meat with a spatula. Remove from heat.
Spread on plate lined with paper towels to cool.
Start charcoal fire or preheat gas grill for direct cooking.
In bowl combined cooled chorizo with ground beef.
Form into six 3/4-inch thick patties.
Indent center of each patty.
Grill over medium heat, covered, for 10 to 12 minutes, until done (160 degrees F), turning once midway through grilling time.
Spread Poblano -Avocado Spread on buns.
Layer patties and pickled onions on buns. Makes 6 burgers.