In a small bowl, combine the pesto, water, balsamic vinegar, garlic and sweetener. Season with a little salt and pepper.
Add the chicken to a bowl and pour over half the pesto dressing. Coat the chicken, cover and pop into the fridge for a minimum of 30 minutes. If you have time, leave it longer to allow the flavours to develop further.
Add the pine nuts to a small frying pan and set on a low heat. Heat for 2 minutes until the pine nuts are lightly toasted and golden. Leave to one side.
Assemble the salad. Divide the lettuce, tomatoes and cucumber between two plates. Crumble over the feta cheese and sprinkle over the pine nuts.
Once the chicken has marinated, spray a frying pan with low-calorie cooking spray. Set on a medium heat and add the chicken to the pan. Cook for 10 minutes, moving the chicken round the pan to brown on all sides. Check that the chicken is cooked through, and no pinkness remains.
Add the chicken to the top of the salad, drizzle over the remaining dressing and serve.