1. Preheat the oven to 190C/375F/gas mark 5. Make the marinade by mixing the oil, lemon juice, thyme, tarragon, garlic and seasoning together.
2. Cut 3 slits in each chicken breast and arrange them skin side up in a suitable ovenproof dish, in a single layer. Pour the marinade over the chicken, dividing it equally among the 4 breasts. Allow the marinade to soak into the flesh for 30 minutes.
3. Place the marinated chicken in the centre of the preheated oven and cook for 20-25 minutes or until cooked through.
4. Remove the cooked chicken from the oven, then preheat the grill to moderate. Slice the ham in half to give 4 pieces and arrange one piece on each breast. Top with the cheese and 2 slices of tomato, then season to taste.
5. Place under the grill for 6-8 minutes or until the cheese starts to bubble. Serve immediately with a green salad and sautéed potatoes.