Peel, wash and dry the potatoes. Cut them into thin slices. Heat the butter and a tablespoon of oil in a large non-stick frying pan. Add the potato slices and cook over a low heat with the lid on for 10 minutes and then remove the lid, turn up the heat and sprinkle with the herbes de Provence and cook for a further 5 minutes stirring constantly. Drain the potatoes. Preheat the oven to gas mark 6, 180°C and oil a rectangular cake tin. Beat the eggs and season with salt and pepper. Cut the slices of ham into strips and then into small pieces. Place the potatoes into the base of the tin and then pour the beaten eggs over the top. Bake in the oven for around 15 minutes. Meanwhile, heat a tablespoon of oil in a frying pan. Add the pieces of ham and brown them and then drain them on kitchen paper. Take the tortilla out of the oven and turn it out onto a board. Cut it into small squares. Arrange them on a plate and top them with the pieces of fried ham. Serve immediately.