Get Ready: Preheat your oven to 350°F (175°C). Lightly spray a 3-quart baking dish with nonstick cooking spray and set it aside.
Brown the Meat: In a large skillet, cook the lean ground beef and diced onion over medium-high heat for about 4 to 6 minutes, until the beef is fully browned and the onions are soft. Make sure to drain any excess fat for a healthier dish.
Add Flavor: Stir in the minced garlic and cook for another minute or two until it smells wonderful. Then, add the ketchup, Dijon mustard, and relish (if using), mixing everything thoroughly. Cook for an additional 1 to 2 minutes until fragrant. Remove the skillet from the heat.
Combine It All: Stir in the undrained diced tomatoes, crumbled bacon (if you're using it), 1 ½ cups of the shredded cheese, and the half-and-half (or Greek yogurt).
Mix in Pasta: Gently fold in the cooked and drained rotini pasta until everything is well combined. If your skillet is too full, transfer the mixture to your prepared baking dish or a large bowl before mixing the pasta.
Top with Cheese: Sprinkle the remaining ½ cup of shredded cheese evenly over the top of the casserole.
Bake to Perfection: Cover the baking dish with aluminum foil. Bake for 15 to 25 minutes, or until the cheese is completely melted and bubbly, and the casserole is heated through.
Serve: Remove from the oven, uncover, and let it rest for a few minutes. Garnish with fresh chopped green onions, if desired, and serve hot!