Simple Two-Potato Gratin

Name: Simple Two-Potato Gratin
Yield: 8 Servings
Ingredients: 3 pounds peeled and very thinly sliced russet and sweet potatoes (use a V-slicer if you have one); Butter (for buttering baking dish and foil); 1 1/2 cups heavy whipping cream; 1/2 cup chicken broth; 1 tablespoon chopped sage; 1 minced garlic clove; 1 teaspoon kosher salt; Ground black pepper; 1 cup grated Gruyère cheese