Cheesy Onion Bun In N Out Style Burgers

1. Cut an onion into thin slices. 2. Place each onion slice between two piles of shredded cheddar on a parchment-lined baking sheet. 3. Bake at 400F (200C) for 6-10 minutes until golden and crispy. 4. Meanwhile, mix 80/20 ground beef with salt and pepper. 5. Flatten the mixture into patties and rub each side with mustard. 6. Fry the patties over medium heat until cooked to your preference. 7. For the spread, mix 1/4 cup mayo, 1.5 tbsp sugar free ketchup, 2 tbsp dill relish, and 1 tsp vinegar. 8. Assemble your burgers with the cheesy onion buns, burger patties, and special spread.