Boil the potatoes unpeeled in salted water for 18-20 minutes. Pour into a sieve, let it steam out and peel while still warm and cut in half lengthways.
Melt the butter with the lemon zest and 2 tablespoons of olive oil in a saucepan and set aside.
Heat the remaining oil in a pan and fry the potatoes in it over medium to high heat for 6 - 8 minutes until crispy. Add the tomatoes and sage leaves and fry briefly until the skin of the tomatoes cracks easily. Season with a little salt and pepper. Add the lemon butter and lemon juice and heat with the tomato potatoes.