Slice the flank steak against the grain into 16 to 20 strips; they should be between ⅛-inch to ¼-inch thick.
Season the steak slices with cumin, salt, and pepper.
Thread the steak onto the skewers, accordion style. Set aside.
In a blender combine olive oil, chopped cilantro, garlic, red wine vinegar, lemon juice, pepper flakes, salt, and pepper. Pulse until thoroughly combined, then set aside.
Brush the grill grates with olive oil.
Place steak skewers on the grill and cook for about 1 minute per side, or until browned and grill marks appear.
Remove steak skewers from grill.
Brush steak with chimichurri sauce.
Garnish with red pepper flakes, if you like, and serve.