Fill a medium pan with salted water, place over a high heat and bring to a boil. Peel and finely slice the garlic and onion, then peel and finely grate the ginger. Halve and deseed the chilli, then finely slice. Roughly chop the tomatoes. Add the rice to the boiling water and cook for 10 minutes, or until tender. Drain in a sieve, then return to the warm pan, cover with a lid and set aside off the heat. Heat 2 tablespoons of oil in a medium pan over a medium heat, add the mustard seeds, garam masala and turmeric, then fry for 1 minute. Add the onion, garlic and ginger, then fry for 10 minutes, or until softened. Add the chilli, tomatoes and a splash of water, then turn the heat up and cook for a further 3 minutes. Next, add the prawns and coconut milk, bring to the boil then reduce the heat slightly and simmer gently for 3 to 4 minutes, or until the prawns are just cooked. Stir through the spinach and cook for a further 1 to 2 minutes until the spinach has wilted. Divide the rice between plates. Season the prawn curry to taste with salt, black pepper and a squeeze of lime juice, then ladle the curry onto the rice. Serve with the remaining lime, cut into wedges for squeezing over.