Steak sandwiches with sweet potato fries

Method

1. Heat the oven to 200°C/180°C fan/gas 6. On a baking tray, season the sweet potato chips and toss with 1 tbsp oil. Bake for 30 minutes until tender but crisp on the outside, tossing once.
2. Meanwhile, heat a glug of oil in a frying pan over a medium heat. Cook
   the onion with a pinch of salt for 10-12 minutes, stirring, until soft and starting to brown. Add the sugar and balsamic vinegar, then bubble for 3-4 minutes until caramelised. Remove from the heat and set aside.
3. Brush the steaks with oil and season well on both sides. Heat a griddle pan until smoking hot, then fry the steaks for 2-3 minutes on each side or until done to your liking. Remove to a board, cover with foil and leave to rest for 5 minutes.
4. Wipe the griddle pan clean, then toast the ciabatta rolls, cut-side down, for 3-4 minutes until crisp. Remove and spread each bottom half with 1 tsp dijon mustard.
5. In a bowl, toss the rocket with a drizzle of oil, a squeeze of lemon juice and some black pepper. Divide between the ciabatta bases. Slice the steak into strips and arrange over the rocket, then top with the onions and ciabatta bun tops.
6. In a bowl, mix the mayonnaise with the remaining mustard and a little black pepper. Serve the steak sandwiches with the sweet potato fries and mustard mayo on the side.