Put the red chillies and coriander seeds in a mortar and pound with a pestle or grind in a spice grinder. Set aside.
Heat the ghee or oil in a kadhai or wok, add the onions and saute over medium heat for about 5 minutes, or until golden brown. Add the garlic paste and stir-fry for 15 seconds, then add the pounded spices, ajwain seeds, and stir-fry for a further 30 seconds.
Add the tomatoes, then increase the heat to high and add three-quarters of the ginger and chillies. Reduce the heat to medium and cook for about 5 minutes, or until the oil separates from the sauce.
Add the prawns and continue to cook for a further 5 minutes, or until the prawns are cooked. If there is some liquid left, then increase the heat to high and cook for a further 1-2 minutes until the liquid has evaporated.
Season with salt, then sprinkle over the garam masala, fenugreek, lemon juice, and coriander. Stir to combine and serve hot.