Preheat your oven to 350°F (175°C) and lightly grease a baking dish. In a large skillet over medium heat, cook the ground beef and chopped onion until the beef is browned and the onion is soft, about 5–7 minutes. Stir in the taco seasoning and cook for another minute or two, seasoning with salt and pepper to taste. In a medium bowl, whisk together the eggs, heavy cream, and a bit more salt and pepper until smooth. Spread the cooked beef mixture evenly in the bottom of the prepared baking dish, then pour the egg mixture over the top. Sprinkle shredded cheddar cheese evenly over everything. Bake uncovered for 25–30 minutes, or until the casserole is set and the cheese is melted and bubbly. Let cool slightly before serving, then top with salsa or serve it on the side. Enjoy warm!