1. Heat oven to 400 degrees . In a small bowl, mix together chopped garlic, rosemary and thyme. In a 2-quart baking dish, layer one-third of the sweet potato and baking potato slices, slightly overlapping some of the edges. Sprinkle with 1/4 tsp of the salt, 1/8 tsp of the pepper, half the garlic-herb mixture and one-third of the shredded cheese. Repeat layering a second and third time. Mix remaining 1/2 tsp salt with heavy cream; pour over potatoes.
2. Place baking dish on a rimmed baking sheet. Cover with aluminum foil. Bake at 400 degrees for 30 minutes. Uncover and bake another 25 minutes, until bubbling and top is golden-brown. Cool 15 minutes before serving.