In a small bowl, whisk together the Greek yogurt, sugar-free ketchup, yellow mustard, garlic powder, and onion powder to create the secret sauce.
Divide the ground beef into two equal portions and shape them into thin patties, then season both sides evenly with sea salt and black pepper.
Place the bacon slices in a cold cast-iron skillet and turn the heat to medium, cooking until the fat has rendered and the bacon is crispy.
Remove the bacon to a paper towel-lined plate, leaving a small amount of the rendered fat in the skillet.
Increase the heat to medium-high and add the beef patties to the skillet, searing for 3 minutes until a deep brown crust forms.
Flip the patties and place the cheddar cheese slice on one of them, then cover the pan with a lid for 1 minute to melt the cheese while the second side finishes.
Lightly toast the sprouted grain bun in the skillet or a toaster until golden.
Assemble the burger by spreading the secret sauce on both sides of the bun, then layering the lettuce, tomato, onion, both beef patties, and the crispy bacon.