Heat Oil In Skillet Over Moderate Heat And Fry Pork For 5 Minutes. Add Water And Bring To A Boil. Prepare A Smooth Sauce In Processor With The Onion, Sesame Seeds, Pepper, Salt, Cumin, Achiote And Tomatoes. Add To Pork And Simmer Over Moderate Heat For 45 Minutes. Add Potato And Simmer For 10 Minutes. Add Flour Mixture And Stir Well. Simmer For 15 Minutes. Serve Warm With Tortillas.