Cut the bread at 1.5 cm (¾ inch) intervals, leaving the slices only just attached at the base.
Snip the chives with scissors and mix half in a bowl with the garlic butter. Roughly spread a dot of butter into each cut of the bread. Lay the loaf on a piece of foil and bring the edges up over the top, scrunching them together. Bake in a preheated oven, 220°C (425°F), Gas Mark 7, for 10 minutes.
Meanwhile, heat 1 tablespoon of the oil in a large frying pan. Add the chicken and fry gently for 5 minutes until golden on the underside (check by lifting a piece with a fish slice). Turn the pieces over and fry for a further 5 minutes.
Using oven gloves, carefully open out the foil on the garlic bread and bake for a further 10 minutes.
Scatter the tomato and mozzarella slices in a shallow dish.
Make a dressing by whisking together the remaining oil with the vinegar, mustard and sugar in a bowl. Scatter the remaining chives over the salad and drizzle with the dressing. Using oven gloves, remove the garlic bread from the oven and serve with the salad and chicken.