In a large skillet over medium-high heat, brown the ground beef until fully cooked and no longer pink, breaking it into small crumbles with a spatula.
Season the beef with salt, black pepper, garlic powder, and onion powder, then drain any excess fat from the pan.
In a small mixing bowl, combine the Greek yogurt, yellow mustard, sugar-free ketchup, and sweet pickle relish to create the sauce.
Divide the shredded romaine lettuce equally between two large serving bowls.
Layer the seasoned beef, cherry tomatoes, red onion, pickles, and shredded cheddar cheese over the lettuce.
Top each bowl with avocado slices and drizzle with the prepared Greek yogurt sauce immediately.