Brush the pork loin steaks with Dijon mustard and set aside to marinate for at least 20 minutes, or up to 8 hours in the fridge.
Heat the oil in a large non-stick frying pan over medium heat. Add the sliced red onion and cook gently for 5 minutes, until softened.
Push the onions to one side of the pan. Add the pork steaks and cook for 30 seconds, then flip and cook for about a minute. Turn them once more.
Stir in the chopped garlic and cook for 1 minute, then pour in the cream. Season with salt and pepper, and let it bubble for a minute.
Stir in the kale, cover with a lid and reduce the heat. Simmer for about 5 minutes, until the kale has wilted, the pork is tender and the mustard sauce is thick and creamy.
Transfer the pork steaks to plates, spoon over the mustard and kale sauce, and serve hot with mashed or boiled potatoes.