In a small bowl, stir together oil, soy sauce, lemon juice, garlic and pepper.
Slice steak thinly across the grain. Cut with the knife at a 45 degree angle so that you get strips that are wider then they would be if you cut straight down.
Combine the steak strips and marinade in a large ziptop bag and refrigerate for at least 8 hours.
To make sauce, bring soy sauce, brown sugar, garlic, vinegar, chili sauce, ginger, and black pepper to a boil in a small saucepan.
In a small bowl, stir together water and cornstarch. Add to saucepan and simmer sauce until thickened, about 2 to 3 minutes.
Tread steak onto skewers. Grill over high heat for 3 minutes. Flip over and grill 1 to 2 more minutes. Serve with green onions, sesame seeds, and sauce.
Notes
For easy slicing, place flank steak in the freezer for 20 minutes before slicing.