Combine olive oil, 1/4 teaspoon kosher salt, chipotle chile powder, and black pepper; rub evenly over top sirloin. Cut steak into 32 cubes. Thread steak, avocado, cherry tomatoes, and onion alternately onto skewers. Coat with cooking spray; sprinkle with 1/4 teaspoon kosher salt. Place skewers on a grill rack coated with cooking spray. Grill 5 minutes or until done, turning skewers occasionally for an even char.
Note: If using wooden skewers, soak them in water for 20 minutes before grilling.
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