Preheat skillet to medium heat. Place frozen burger into skillet and cook on one side until juices begin to come through. Flip the burger and continue to cook until internal temperature reaches 160°F.
While burger patties are cooking, prepare the Whole30 ‘Animal Sauce’ by mixing mayonnaise, ketchup, relish, coconut aminos, and vinegar in a small bowl, whisking to coming. Set aside until ready to use.
Set another skillet over medium heat and add avocado oil. Once warm, add chopped onions, salt and pepper, and cook until onions are brown and caramelized, stirring occasionally to prevent burning, about 10-15 minutes. Set aside.
Slice lettuce heads in half and pull out inner layers until you are left with outside layers (bun) to wrap around burger patties.
Add burger patty, sliced tomatoes, caramelized onions, and homemade animal sauce to the lettuce bun.
Sit back, snap a pic to share, and enjoy!