Cut slits on both sides of the fish 2cm apart. Set aside.
To make the marinade, place coriander (cilantro) leaves, ginger and green chili in a mortar and pestle. Make a coarse paste. Combine together coriander paste, ground turmeric, ground cumin, black pepper, red chilli powder, oil and lemon juice in a small bowl. Season with salt.
To marinate the fish, place the lemon slices inside the fish. Place the garlic cloves in the slits. Rub the marinade onto the fish. Cover and refrigerate overnight or at least 2 hours.
When fish is ready, preheat the grill. Place the fish on the grill, and cook for 20 minutes. When one side is done, flip over to cook the other side.
Serve with onion salad or challas.