In a small saucepan, combine rice vinegar, water, soy sauce, brown sugar, oil, flour, ginger, garlic, and sesame seeds. Whisk the sauce together and place over medium heat. Simmer until the sauce is slightly thickened, 8 to 10 minutes. Keep warm.
The steak:
Season flank steak with salt and pepper.
Heat a grill to medium-high heat. Coat the grates with oil.
Cook until the steak is medium rare (about 140 degrees F in the center), about 7 minutes per side. Let rest for 10 minutes before slicing across the grain into ¼-inch thick slices.
Notes
Please note that I am not a medical or nutritional professional. I provide nutritional information for my recipes as a courtesy to my readers. It is calculated using the Sparks nutrition calculator and Weight Watchers points are calculated using the Recipe Builder on their site. While I attempt to provide information that is as accurate as possible, you should calculate the nutritional information independently before relying on it.