Preparation
Mix all the ingredients for your mustard mayo together. Set to the side.
Place all the glaze ingredients in a pan, bring to the boil and simmer for 10 minutes. Strain through a fine sieve onto a large tray to cool it down as quickly as possible. Transfer to a non-reactive container that will fit in the fridge. Pop the steaks in the glaze and marinade for 4 hours.
For the onions, place the oil in a heavy-bottomed pan over a high heat and bring to a smoking heat. Carefully add the onions (watch you don’t get splashed by the spitting fat) and let them sizzle in the oil until they start to char, then turn them over until you have a good amount of charring on both sides, taking care not to burn them. Turn down the heat, add the rest of the ingredients and slowly cook with a lid on for 20 minutes. Season.
For the potatoes, place the potatoes in a pan of cold water, add salt, then cover and bring to the boil. Turn the heat to a simmer and cook the potatoes until they are done – they should slip off the tip of a knife. Drain, pop back into the pan with the parsley and season with salt and pepper.
Take out the steaks and pat dry. Pour the marinade into a pan, bring to the boil, and reduce by half, until a syrup consistency.
Heat a griddle, frying pan, or even a barbecue until hot. Cook the steaks (you’ll probably need to do them in batches) and once you have a nice colour on one side (after about 3 minutes), flip over and cook the other side. Brush the browned side with the reduced glaze. When ready, remove from the pan, brush the other side of the steaks with the glaze and let them rest on a plate, covered with foil, for 5 minutes.
To serve, place each glazed steak on a plate with a pile of onions and some potatoes. Garnish with the parsley and serve with mustard mayo and an extra drizzle of the glaze.