Mix the oil, thyme, lemon juice, garlic and salt in a bowl. Add the chicken stir well to coat evenly. Leave to marinade, 1 hr.
Preheat the oven 180 C. Spread the bread cubes in a single layer on a baking tray, then brush all over with the oil. Bake for about 10 mins until golden and crisp. At the same time cook the chicken on a baking tray lined with baking paper, cook for about 20 minutes. Leave for 5 mins to rest.
Meanwhile, put the lemon juice, garlic, mustard powder and the oil in a small bowl, stir together. Set aside.
To serve, roughly chop the lettuce and chicken, toss together, with the croutons and dressing. Spoon onto serving plates.