Preheat a gas grill to high or prepare coals for direct-heat cooking on charcoal grill. Form ground beef
into 3 equal patties without over working and making the center slightly thinner than the edges. Rub each patty
with the chili powder and season with salt and pepper. Grill
over high heat, approximately 5 minutes. Turn the burgers over and cover with plenty of Cheddar cheese. Close the lid and continue grilling until the cheese melts, another 4 minutes.
Meanwhile, prepare the onions by placing one of the sliced onions in a large skillet over medium heat with 2 tablespoons canola oil
. Season with salt and pepper and let the onions saute for approximately 30 minutes, tossing occasionally, or until they are lightly brown and caramelized. Season with salt and pepper.
Combine the Dijon mustard, whole grain mustard, sour cream, horseradish, Worcestershire sauce
, and salt and pepper in a bowl.
Assemble by placing each patty on a bun and top with a raw onion round, sauteed
onion, and spread about a spoonful mustard sauce on top. Serve the remaining mustard sauce alongside the burgers if more is desired.