Heat oven to 200C/180C fan/gas 6. Place the chips on a non-stick baking tray, toss with oil and Cajun seasoning, then spread out in a single layer. Pop in the oven and bake, turning once, for 20-25 mins or until golden and crisp. Season.
Meanwhile, mix the mayonnaise, chives and mustard in a small bowl and season. Spread the mix onto 2 of the slices of bread. Layer 1 spread slice with radishes. Place the other slice, spread-side up, on top and add the salt beef and lettuce. Top with the final slice.
Cut up and secure with cocktail sticks if you like, then serve alongside the fries.