Cook the kale in a large saucepan of lightly salted boiling water for 3 minutes. Meanwhile, heat the oil in a nonstick frying pan and cook the onion for 2 minutes until softened.
Stir the mustard into the vegetable mash and add to the pan. Add the drained kale, season to taste and mix well. Cook over a medium heat for 3 minutes on one side until golden, then flip the mixture over and cook for a further 2 minutes until piping hot. Serve immediately.