Preheat the oven to 200 degrees. Wash the tomatoes, remove the stem and cut the tomatoes into slices. Season the tomato slices well on one side with salt and pepper. Butter a gratin dish and layer the tomato slices so that they overlap.
Whisk the cream, milk and egg well, stir in the cheese and crushed garlic and season the topping with salt, pepper and nutmeg.
Pour over the tomatoes and bake the gratin in the oven on the second groove from the bottom for 20 to 25 minutes. Take out of the oven and serve sprinkled with the chives.