In a small saucepan over medium heat, combine cream, milk, butter, and minced garlic.
Bring to a simmer, then remove from heat.
Stir in parsley, thyme, sage, rosemary, salt, and pepper.
Arrange half of the sweet potatoes in the bottom of the prepared baking dish.
Pour half of the cream mixture over the sweet potatoes and sprinkle with half of the Gruyere cheese.
Repeat the layers once more, and then cover the dish with aluminum foil.
Bake in the preheated oven for 30 minutes.
Remove the foil and continue baking, uncovered, until the sweet potatoes are tender and the cream and cheese mixture is bubbly and golden brown, about 15 minutes.
Remove from the oven and let it sit for 10 minutes before serving.
