In a medium bowl, gently combine the ground beef with sea salt, black pepper, and garlic powder, being careful not to overwork the meat.
Form the mixture into a single thick patty, pressing a small indentation into the center with your thumb to ensure even cooking.
Heat a cast-iron skillet or grill pan over medium-high heat until very hot.
Place the patty in the pan and sear for 4-5 minutes per side for medium doneness, or until it reaches your desired temperature.
During the last minute of cooking, place the cheddar cheese slice on the patty and cover the pan briefly to melt it.
While the meat rests, lightly toast the whole wheat bun in the pan or a toaster until the edges are golden brown.
Spread the yellow mustard on the bottom bun, then layer the lettuce, tomato, and red onion.
Place the cheesy beef patty on top of the vegetables and close the burger with the top bun.