Peel the potatoes and cut into 3 - 4 mm thick slices. Cook in salted water or vegetable stock for about 5 minutes.
Let cool down a little and layer in a casserole dish that has been greased with butter. Chop the lovage and spread on top. Season with salt and pepper.
Cut the tomatoes into slices and place them on top (but you can also omit or add kohlrabi slices, for example). Pour the cream (sour cream also tastes good) and cover with flakes of butter and the Gruyère or other cheese.
Bake for about 45 minutes at 175 degrees top / bottom heat.