Preheat your oven to 375°F (190°C). Lightly grease a 9x13 inch baking dish or an equivalent 3-quart casserole dish.

Brown the Beef: In a large skillet over medium-high heat, brown the ground beef along with the finely chopped onion. Drain off any excess fat once the beef is fully browned.

Sauté Aromatics: Reduce the heat to medium. Add the minced garlic to the skillet and cook for 1 minute until fragrant.

Simmer the Base: Stir in the diced tomatoes with green chilies (undrained), beef broth, sugar-free ketchup, yellow mustard, dried oregano, salt, pepper, and red pepper flakes (if using). Bring the mixture to a simmer and cook for 5 to 7 minutes, allowing the liquid to reduce slightly and the flavors to meld.

Create the Creamy Topping: While the beef mixture simmers, prepare the creamy layer. In a medium bowl, combine the softened cream cheese, mayonnaise, and half (1 cup) of the shredded cheddar cheese. Mix thoroughly until smooth and well combined.

Layer: Pour the seasoned ground beef mixture evenly into the prepared baking dish. Spread it out flat.

Add Topping and Bacon: Evenly drop spoonfuls of the cream cheese and mayonnaise mixture over the top of the beef. Gently spread the creamy topping to cover the beef entirely. Sprinkle the crumbled bacon evenly over the creamy layer.

Add Final Cheese Layer: Sprinkle the remaining 1 cup of shredded cheddar cheese over the top of the casserole.

Bake: Bake for 20 to 25 minutes, or until the cheese is melted, bubbly, and lightly browned on the edges, and the casserole is heated through.

Rest and Serve: Remove the casserole from the oven and let it rest for 5 to 10 minutes before slicing and serving.