For the Spice Blend, mix all the ingredients until well blended. Reserve 1 tbs for the Tomato Onion Sauce.
For the Beef Skewers, mix the remaining Spice Blend, tomatoes, papaya, lemon juice and oil in a food processor or blender. Process the mixture until smooth. Pour the mixture into a large resealable plastic bag. Add the beef cubes and toss to coat well. Refrigerate for 1 hour.
Meanwhile, for the Tomato Onion Sauce, heat the oil in a medium saucepan on medium heat. Add the onion and cook and stir for 5 - 6 minutes or until softened. Add the reserved Spice Blend and remaining ingredients and mix well. Bring to the boil and then reduce the heat to low and simmer for 10 minutes, stirring occasionally. Keep warm until needed.
Remove the beef from the marinade and discard any remaining marinade. Thread the beef onto skewers. Grill over a medium-high heat for 3 - 4 minutes per side or until cooked as required. Serve with the Tomato Onion Sauce and warm flatbread. If desired, serve with a side of East African Fresh Tomato & Cucumber Salad.