To make the toast, heat a griddle pan until smoking hot. Spread mayonnaise on the top half of the bread bun and serve with potato chips. Drizzle the sliced ciabatta with the olive oil, season and then griddle for 1–2 minutes until golden on both sides. Combine the ingredients for the mustard mayonnaise. Season, then spoon into a serving bowl and set aside. Top with thinly sliced slow roasted beef, followed by the caramelized onions and peach relish. Layer the salad leaves on top.