1. For the Spice Blend, mix all ingredients until well blended. Reserve 1 teaspoon for the Tomato Onion Sauce.
2. For the Beef Skewers, mix remaining Spice Blend, tomatoes, papaya, lemon juice and oil in food processor or blender. Process until smooth. Pour mixture into large resealable plastic bag. Add beef cubes; toss to coat well. Refrigerate 1 hour.
3. Meanwhile, for the Tomato Onion Sauce, heat oil in medium saucepan on medium heat. Add onion; cook and stir 5 to 6 minutes or until softened. Add reserved Spice Blend and remaining ingredients; mix well. Bring to boil. Reduce heat to low; simmer 10 minutes, stirring occasionally. Keep warm.
4. Remove beef from marinade. Discard any remaining marinade. Thread beef onto skewers. Grill over medium-high heat 3 to 4 minutes per side or until desired doneness. Serve with Tomato Onion Sauce and warm naan bread. If desired, serve with a side of East African Fresh Tomato & Cucumber Salad (see separate recipe).
Test Kitchen Tips:
Tamarind concentrate or paste is a thick fruit pulp that is tart, slightly sweet and fruity. Look for it in the Asian or Mexican aisle of grocery stores or online.
Soak wooden skewers in water for at least 30 minutes. This prevents them from burning on the grill.