Cut the chicken in bite-sized pieces. Place them in a bowl and add the oil, lemon and garlic. Give everything a good toss, cover the bowl and put it in the fridge to marinate for at least 1 hour. Warm the oil in a deep frying pan and wilt the onions for 2-3 minutes. Add the marinated chicken along with the marinade and cook for 5-6 minutes. Remove the chicken from the pan and place it in a platter. Add the peppers, aubergine and courgettes back to the pan and wilt for 3-4 minutes. Now put the chicken back to the pan, add the tomatoes, sprinkle with the oregano and season with salt and pepper. Cook for 5 more minutes and enjoy!