Preheat oven to 425F
In a saucepan, bring heavy cream, garlic, rosemary, thyme, and salt to a simmer over low-medium heat. Stir, then turn off heat and cover
Thinly slice the sweet potatoes using a knife or mandoline
Butter a 9x9 baking dish. Add a layer of sweet potatoes and cover with ½ cup cheese. Repeat until you have 4 layers
Pour strained steeped heavy cream over final layer of sweet potatoes, then add final layer of cheese. Cover with foil and bake for 40 minutes on middle rack
Uncover and bake for another 15 minutes on top rack. If needed, broil on high for 2 minutes until golden brown and crispy
