Put the mini chicken breast fillets into a plastic bag with half the vinegar and toss together until evenly coated.
Heat a nonstick frying pan, lift the chicken out of the plastic bag with a fork and add the pieces to the pan. Fry for 3 minutes, turn and drizzle with the vinegar from the bag and cook for 2–3 more minutes or until browned and cooked through.
Toast the bread lightly on both sides. Slice the chicken into long, thin strips and arrange them on 4 slices of toast. Mix together the yogurt and horseradish and a little pepper to taste. Add the salad leaves and toss together.
Spoon the yogurt and salad leaves over the chicken, drizzle over the remaining vinegar, if liked, and top with the remaining slices of toast. Cut each sandwich in half and serve immediately.