Heat the oil in a kadai; add the mustard seeds and fennel seeds and sauté till the seeds begin to splutter.
Add the onion and sauté until they begin to colour.
Add the ginger paste and garlic paste and sauté for one minute. Add the coriander powder and chilli powder and sauté for one minute longer.
Add the tomato purée and continue to cook till the oil separates.
Add the spinach and cook till the leaves begins to wilt. Add the prawns and salt. Cook, uncovered, for six or seven minutes.
Stir in the garam masala powder and cook for one more minute. Serve hot.