In a large bowl, combine the soy sauce, oyster sauce, sweet soy sauce, ground coriander, salt, and black pepper.
Add the beef or chicken pieces to the marinade, ensuring they are well coated. Cover and refrigerate for at least 1 hour, or overnight for best results.
Thread the marinated meat onto the skewers.
Preheat your grill to medium-high heat.
Grill the skewers, turning occasionally, until the meat is cooked through and nicely charred on the outside, about 10-15 minutes.
While the skewers are grilling, heat the vegetable oil in a skillet over medium heat.
Add the finely chopped onion and sauté until translucent, about 5 minutes.
Stir in the minced garlic and fresh ginger, cooking for another minute.
Add the chopped tomatoes and cook until they soften, about 3-4 minutes.
Stir in any remaining marinade and cook until the sauce thickens slightly.
Remove the skewers from the grill and let them rest for a few minutes.
Enjoy your meal!