Grind the 2 types of meat and put them in a deep bowl. Add the grated onions, garlic and the saffron that's been soaked in the hot water for several minutes, as well as the baking soda. Season with salt and black pepper.
Form small meatballs, flatten them a little and put on skewers. Cut the tomatoes in half and put them on separate skewers.
Bake on a heated grill that's been smeared with butter ahead of time.