Preheat your oven to 375°F (190°C). Lightly grease a 9x9-inch baking dish or similar-sized casserole dish.
Heat the olive oil or butter in a large skillet over medium heat. Add the diced onion and sauté for 2-3 minutes until softened. Stir in the minced garlic and cook for 1 minute until fragrant.
Add the ground beef to the skillet, breaking it up with a spoon. Cook for 6-8 minutes, stirring occasionally, until browned and fully cooked. Drain excess fat if needed, then season with salt and black pepper.
In a medium bowl, whisk together the heavy cream, eggs, sugar-free ketchup, yellow mustard, and Worcestershire sauce until smooth. Stir in half of the shredded cheddar cheese.
Transfer the cooked beef mixture to the prepared baking dish, spreading it evenly. Pour the cream mixture over the beef, then sprinkle the remaining cheddar cheese on top. If using diced pickles or extra cheese, add them now.
Bake in the preheated oven for 20-25 minutes, until the cheese is melted and bubbly and the casserole is set. Let it cool for 5 minutes before serving. Enjoy your keto cheeseburger casserole hot, with all the flavors of a burger in every bite!