Heat grill or grill pan to medium high. Heat broiler. Combine chili powder, garlic powder and cumin in a small nonstick skillet. Cook over medium-high heat for 2 minutes, until fragrant. Remove from heat.
In a large bowl, stir together the sirloin, black beans, spice mixture, red onion, 1/3 cup of the salsa, the salt and pepper. Transfer to a food processor and pulse until texture is slightly smooth and about half of the beans have been mashed. Return to bowl and divide into 6 hamburger patties.
Coat burgers with nonstick cooking spray. Grill 4 minutes, then flip over and grill another 4 minutes or until instant-read thermometer inserted into thickest part of burger registers 140°.
Meanwhile, toast buns under broiler for 3 minutes, turning once. In a small bowl, stir together remaining 3 tablespoons salsa and the mayonnaise. Spread each bun with some of the salsa mayo. Add a burger and a few avocado slices and serve.