Cook bacon over medium-high heat until crisp, about 6 minutes. Drain. Cut strips in half. Gently mix ground chuck, ground sirloin, salt, and pepper. Shape into 6 burgers, about 1-inch thick. Grill over high heat, turning once or twice and brushing occasionally with butter, for about 9 minutes (internal temperature should reach 160°F). Top burgers with cheese during last minute of cooking. Let cheese melt. Spread Russian dressing on both top and bottom buns. Put lettuce and tomatoes on bottom buns. Top with burgers, onions, and bacon. Serve immediately. Serves 6.