Mix the marinade ingredients together in a small bowl, then spread evenly onto each side of the chicken pieces. Set aside to marinate.
Mix the pesto mayonnaise ingredients together in a bowl.
Warm a pan on medium heat, butter your sourdough slices on both sides and toast them in the pan until golden, then remove. This gives crunch and colour, and prevents any juicy ingredients from making the bread soggy.
Heat olive oil in the same pan over medium heat. Place chicken breast in the pan, cook for 4 minutes or until golden brown, turn to the other side and cook for another 3-4 minutes, or until the internal temperature reaches 165°F. Remove onto a plate.
Spread the pesto mayonnaise on all the slices. Layer the sandwich in the following order: sourdough slice, sun-dried tomatoes, chicken, basil, cheese, spinach. Top with the second slice of sourdough bread and serve.