Boil green beans 3-4 minutes in lightly salted water. They should remain crispy.
Heat the oil in a frying pan and put in the mustard seeds. As soon as they start to sauté add the chopped onion. Let it cook 1 minute or 2 and add the spices. Stir well, add a trickle of water and let cook on low heat for 2 to 3 minutes.
Add the fennel, cover 5 minutes and add the green beans. Stir well and leave in pan again 2-3 minutes on low heat.
Add pepper and if you want fresh cilantro.