In a skillet over medium-high heat, brown the lean ground beef until fully cooked, then drain any excess fat.
Transfer the cooked beef to the slow cooker basin.
Add the rinsed quinoa, black beans, corn, diced onion, bell pepper, beef broth, salsa, and taco seasoning to the slow cooker.
Stir the mixture thoroughly to ensure the quinoa is submerged in the liquid.
Cover and cook on high for 3 to 4 hours or on low for 6 to 8 hours until the quinoa is tender and the liquid is fully absorbed.
Turn off the heat and sprinkle the shredded cheddar cheese evenly over the top of the casserole.
Replace the lid for 5 to 10 minutes until the cheese is completely melted.
Serve each portion with a dollop of non-fat Greek yogurt to increase total protein content.