Finely slice the onions and heat the oil in a deep frying pan, then gently fry for five minutes or until soft. Crush in the garlic, add the garam masala, ground coriander and cumin, fry for two-three minutes. Add the chopped tomatoes and simmer for 10 minutes. Stir through the spinach and simmer for five minutes more. Stir through the prawns and yogurt and simmer for five minutes. Season with pepper and a good pinch of salt, then cook for a further five minutes or until the prawns are pink and cooked through, and the sauce thickens a bit. Serve with steamed basmati rice, fresh coriander and extra Greek yogurt.