Grilled flank steaks with salsa verde and 12-hour potato stacks

Make the potato stacks. Preheat the oven to 150C. Line an 8-inch square baking tin with baking paper, with some paper hanging over. Toss the sliced potatoes, vegetable oil, chopped rosemary, and 1 tbsp salt in a large bowl. Place a single layer in the prepared tin then top with another layer, covering any gaps. Repeat with the remaining potato. Drizzle any remaining oil over the top, then cover with an 8-inch square piece of baking paper. Press onto the potatoes then covers the pan with foil. Bake for 2-3 hours.
 

Transfer to a wire rack and remove the foil (keep the paper on top). Put another square tin on top and weigh it down with cans. Cool to room temperature then chill in the fridge overnight (with the cans on top).
 

Remove the cans, pan and top sheet of baking paper. Run a knife around the tin to loosen the potato. Carefully turn out on to a board then remove the remaining baking paper. Cut the stack in to 6 lengthways pieces, then each lengthways in to 4 pieces. Transfer the stacks to a baking sheet lined with baking paper then freeze until solid – this takes about 30 minutes but you can do it well in advance.
 

While the potatoes freeze, heat a 2-inch layer of vegetable oil in your largest heavy-bottomed saucepan. If you have a thermometer you want the oil to reach 190C; if you don’t have a thermometer, lower a small piece of bread in to the oil. If it sizzles and turns golden brown, your oil is hot enough.
 

Working carefully and in batches, fry the potato stacks for 5-6 minutes, turning occasionally, until golden and crispy. (We do this in three batches of eight and keep the remaining stacks in the freezer while we fry.)
 

Use a slotted spoon to carefully transfer to a cooling rack and sprinkle with sea salt.
 

For the salsa verde, finely chop the garlic, herbs, capers, gherkins, anchovy, and olives on a board. (You can do this in a processor if you prefer.) Transfer to a bowl then stirs in the lemon zest and juice, mustard, vinegar, olive oil, and chili flakes. Season with salt and pepper. Set aside until needed.
 

For the steaks, combine the mayonnaise, garlic, and rosemary. Brush over the steaks then leave to marinade in the fridge for at least 1 hour. Bring up to room temperature 20 minutes before cooking.
 

Set a frying pan or griddle pan over a medium-high heat. Brush with the olive oil. Sear the steaks for 4 minutes each side then remove from the heat, cover with foil and leave to rest.
 

Thinly slice the steaks against the grain and serve with the salsa verde, potato stacks and broccoli, if you like.