Preheat your oven to 350°F. Cut your onions into your desired size. We like medium sized strips or rounds for this recipe, but it's up to you.
Warm your cast iron over medium heat. Brown your beef in the cast iron with your avocado oil or butter and big pinch of salt and pepper, along with about half of your seasonings, reserving some for your egg mixture. Once beef is browned, add in your bacon bits to crisp up. Then, layer on your onions.
In a large bowl using your hand mixer, beat together your eggs, milk and cream, remaining seasonings, salt and pepper, and tomato paste. Then fold in some of the shredded cheese. Pour the mixture over top of the burger and onions.
Sprinkle on the rest of your shredded cheese, then bake for about 30 minutes at 350°F. You can broil on high at the end at the end if you want the cheese to get a little crispier on top. We love to serve with our homemade thousand island dressing and other yummy burger condiments and dressings.
Nutrition facts do not include thousand island dressing or any additional toppings.