Place tomatillos, onion and garlic on a lightly oiled baking sheet and grill over charcoal, turning often, until onion is crispy and tomatillos have burst - about three to five minutes.
Remove from grill to cool. Once cool, place ingredients in work bowl of a food processor and puree until desired consistency. This can be done up to three days in advance if kept chilled in an air tight container.
Steak
Place flank steak on a sheet of plastic wrap large enough to cover steak once folded. Season steak with salt and pepper. Liberally dust each side of steak with ranch seasoning mix, then sprinkle with red pepper flakes, if using. Wrap tightly in plastic wrap and chill for one to two hours.
Remove steak from refrigerator about 15 minutes before grilling over charcoal.
Unwrap steak and grill over medium heat for five to seven minutes on each side or until an internal temperature of 150°F - allow the steak to rest covered for five minutes. Lightly toast hamburger rolls. Serve steak sliced over rolls and generously topped with salsa verde.