Prep the oven and pan. Preheat the oven to 375°F (190°C).
Lightly grease a 9x13-inch baking dish.
Make the special sauce. In a small bowl, whisk Greek yogurt, light mayo, ketchup, mustard, relish, onion powder, paprika, and vinegar or pickle juice. Season with salt and pepper. Adjust tang and sweetness to taste and set aside.
Brown the meat. Heat a large skillet over medium-high.
Add lean ground beef and break it up. Season with salt, pepper, garlic powder, onion powder, and paprika. Cook until browned, 5–7 minutes.
Drain any excess fat if needed.
Sauté the aromatics. Add diced onion to the skillet and cook until softened, 3–4 minutes. Stir in minced garlic for 30 seconds, until fragrant.
Add cauliflower rice. Stir in the cauliflower rice, broth, and tomato paste. Cook 5–7 minutes until the cauliflower is tender and most liquid has evaporated.
Taste and adjust seasoning.
Layer the casserole. Spread the beef–cauliflower mixture evenly in the baking dish. Drizzle or dollop about half of the special sauce over the top. Sprinkle with half of the cheese.
Bake. Place in the oven and bake 12–15 minutes, until the cheese is melted and the edges bubble slightly.
Finish fresh. Remove from the oven.
Top with shredded lettuce and chopped pickles. Sprinkle with the remaining cheese so it softens slightly from the residual heat. Add sesame seeds if using.
Serve with sauce. Spoon the remaining special sauce over the casserole or serve it on the side so everyone can add more to taste.