For the chicken:
1) Put the vinegar, herbs, mustard, powders if using and oil in a large re-sealable plastic bag. Close the bag and shake to combine all the ingredients. Open the bag, drop the chicken breast into the bag. Close and shake the bag to coat evenly. Freeze for up to 2 weeks.
2) Thaw in the refrigerator overnight, under cold, running water, or in the microwave at 30 percent power for 1 min at a time.
3) Heat a griddle pan. When the griddle is hot, place the chicken on it and cook for about 4 mins per side, or until cooked through. You can also bake the thawed chicken in a 190C/Gas Mark 5 oven for 15 mins, or until cooked through.
For the pesto:
1) Combine the basil, garlic, and pine nuts in a food processor and pulse until coarsely chopped. Add 8 tbsp of the oil and process until fully incorporated and smooth. Season with salt and pepper.
2) If using immediately, add all the remaining oil and pulse until smooth. Transfer the pesto to a large serving bowl and mix in the cheese.
3) If freezing, transfer to an air-tight container and drizzle remaining oil over the top. Freeze for up to 3 months. Thaw and stir in cheese.
For the sandwich:
1) Spread 1 tbsp pesto on 1 side of each slice of bread. Top 4 slices of bread with the marinated chicken. Top the chicken with the roasted peppers. Place the remaining bread slices, pesto-side down on top of the peppers.