First make the pickled cucumbers. Tip the cucumber into a bowl and sprinkle over the salt. Toss well and set aside for 15 mins. Drain the cucumber, gently squeezing any excess water out with your hands, and transfer to a bowl. Heat a small frying pan over a medium-high heat and add the mustard seeds. Toast for 1-2 mins until fragrant and popping, then transfer to the cucumber bowl. Pour over the vinegar, sugar and dill and toss well. Season to taste and set aside. Will keep chilled for up to two days.

Heat the oven to 220C/200C fan/gas 7. Season the steak with 1/2 tbsp of the steak seasoning, season well with salt and pepper, and set aside. Toss the sweet potato and onions with the remaining steak seasoning and the 2 tbsp oil on a baking tray. Arrange in an even layer and season lightly. Roast for 20-25 mins until tender and crispy around the edges.

Meanwhile, mix all the mayo ingredients together and season to taste. Heat a griddle pan over a high heat. Rub a little extra oil over the steak and add to the hot griddle pan. Cook for 1-3 mins on each side, depending on how you like it done – 1-2 mins for medium-rare and 3-4 mins for cooked through. Transfer to a plate and leave to rest.

Split the ciabattas in half and brush the cut sides with oil. Add the ciabattas, cut-side down, to the griddle pan and cook for 1-2 mins until toasted and absorbed the juices from the pan. Set aside.

When ready to serve, thinly slice the steak against the grain. Soak the bottom of the bread in the resting juices, then spread over the mustard mayo. Top with the sweet potato, onions and steak slices, then top with a handful of watercress and some of the pickled cucumbers. Serve with lots of napkins.