2 cups heavy cream
3 cloves garlic, smashed
1 bundle fresh thyme
1/2 stick butter, plus more for greasing
2 pounds turnips, peeled and very thinly sliced
1 1/2 cups grated Parmigiano-Reggiano
1 pound sweet potatoes, peeled and very thinly sliced
A mandolin is the best way to achieve very thin slices of turnips and sweet potatoes.