Place the baguettes on a baking sheet and bake in a preheated oven, 200°C (400°F), Gas Mark 6, for 8–10 minutes, or according to the packet instructions, until crisp.
Meanwhile, lay a chicken breast between 2 sheets of clingfilm and flatten with a rolling pin or meat mallet. Repeat with the remaining chicken breast. Heat a griddle pan over a medium-high heat until hot. Rub the oil over the chicken breasts, season with salt and pepper and cook on the hot griddle for 2–3 minutes or until slightly charred. Turn the chicken breasts over and cook for a further 2–3 minutes or until slightly charred and cooked through but not dry. Remove from the pan, cover with foil and leave to rest.
Make the salad. Mix together the cucumber, mint, lemon juice and a little salt and pepper in a small bowl.
Slice the chicken into thick slices. Cut the baguettes in half lengthways, spread with the red pepper pesto, then fill with the chicken and the cucumber salad. Sprinkle with sunflower seeds and add a few rocket leaves. Cut in half and serve immediately.