Fry the sliced onions in oil and remove. Drain the excess oil in paper towels and set aside. Clean the fish and cut them into small pieces. Rub them with 1-tsp salt, garlic paste and 2 tablespoons of yogurt. Then wash thoroughly. Marinate them with Remaining yogurt, ginger paste, garam masala, ground chilies, salt and fried onions for an hour. Soak the wooden skewers in water for some time and take them out. Then skew the marinated fish in it. Brush oil on the fish pieces and grill them until brown. Brush the remaining marinade half way through the cooking. Take them off the skewers and serve hot with toothpick inserted on them.