In a medium bowl, stir together oil, soy sauce, vinegar, Worcestershire sauce, mustard, garlic, onion powder, black pepper (or red pepper flakes) and cumin. Place meat in a resealable plastic bag and add marinade. Seal bag and gently shake to thoroughly coat the meat. Place the bag in a glass casserole dish and refrigerate for 4–6 hours.
Preheat grill to medium-high heat or set the oven to broil. Remove flank steak from the bag and discard the remaining marinade.
For the grill: oil the grill grate and place steak on the grill and cook 4–5 minutes per side, or to desired doneness. To broil, place steak on slotted broiler pan and broil in oven 5–6 minutes per side, or to desired doneness.
Remove steak from the heat, cover and allow to rest for 5 minutes before slicing thinly against the grain of the meat.
One Last Thing
Slice the steak thinly against the grain for a tender and juicy cut!