In a large bowl, combine the ground chuck and sirloin and mix gently but thoroughly. Divide into eight equal portions and shape them into burger patties (see opposite for tips). Season both sides with salt and pepper and refrigerate for 30 minutes.
Preheat the oven to 350°F.
Heat a large skillet over medium heat. Split and toast the buns in the pan until toasty and golden brown. Set aside.
Remove the burgers from the fridge and place a thumbprint in the center of each patty. Smear 1 teaspoon of the mustard on both sides of each patty (best burger crust ever). Melt the butter in the preheated pan and, working in batches, fry the burgers for 3 to 4 minutes on each side, until a dark brown crust begins to form. The result: perfectly juicy medium-cooked burgers. For folks who prefer their burgers a little more well done, fry for 5 to 6 minutes per side.
Transfer the patties to a baking sheet as they’re done. Top each with cheese and place in the oven to melt, 2 to 3 minutes.
Build the burgers on the toasted buns with 1 or 2 burger patties, a slice of tomato, pickles, a slice of red onion, and lettuce. Top each with ketchup or mayo. Or both. Serve immediately.