Preheat the oven to 400F.
In a bowl mix together ½ cup of heavy cream, garlic clove, rosemary and 2 tablespoons Parmesan cheese. Add the sliced potatoes and season lightly with salt.
Butter the bottom and sides of an ovenproof gratin dish. Layer the potatoes on the bottom and arrange the tomatoes on top. Distribute the remaining 3 tablespoons of heavy cream on the tomatoes. Sprinkle the remaining 2 tablespoons Parmesan cheese over the whole dish.
Cover the potato dish with aluminum foil, bake at 400 for 20 minutes then lower the heat to 350F and cook for another 20 minutes or until the potatoes are tender. Remove the foil and let it cook for another 2 minutes until the top of the tomatoes is golden.