How to Make It
Heat a medium nonstick skillet over medium-high heat. Add ground sirloin to pan; sprinkle with chili powder, cumin, garlic powder, and 1/2 teaspoon salt. Cook until beef is browned, stirring to crumble, about 3 minutes.
Meanwhile, combine beans and water in a small bowl; mash with a fork until smooth. Stir mashed beans into beef mixture; cook 1 minute or until thoroughly heated. Remove beef mixture from pan; cool slightly. Wipe skillet clean.
Cut each bell pepper in half lengthwise. Scoop out membranes and seeds with a spoon. Divide beef mixture evenly among bell pepper halves.
Heat oil in skillet over medium-high heat. Add corn and scallions to pan; sauté 2 minutes. Remove from heat; stir in tomatoes, lime juice, and remaining 1/4 teaspoon salt. Spoon about 1 cup salad into each of 4 portable containers (if making ahead); sprinkle evenly with cheese. Arrange 1/2 cup quinoa in each container. Place 2 stuffed pepper halves in each container. Sprinkle with sliced scallions, if desired. Refrigerate for up to 4 days. Reheat before serving.