To make the patties, divide the sirloin into six 5-ounce sirloin balls and form each ball into a patty. Arrange on a tray, cover, and refrigerate.
Toast the buns and set aside.
Heat a large skillet over medium-high heat and add just enough oil to cover the entire bottom of the skillet. When the oil begins to smoke, reduce the heat to medium and place half of the patties in the skillet. Season the patties with salt and pepper and cook for 3 minutes.
Flip, and cook on the other side for 3 minutes more for medium-rare doneness. Do it again with the remaining patties.
To assemble the burgers, place 1 patty on 1 toasted bun bottom. Top the patty with some chili-Cheddar sauce, 1/4 cup of the sour cream, and 1/4 cup of the scallions. Cover with the top of the bun. Repeat with the remaining ingredients. Wrap half the burger in wax paper. Let rest for 2 to 3 minutes and serve.