Creamy Broccoli Stem Gratin

Name: Creamy Broccoli Stem Gratin
Ingredients: 2 tablespoons unsalted butter, plus extra for buttering the baking dish; 1 large clove garlic, minced; 1 1/4 pounds Yellow Finn or Yukon Gold potatoes, peeled and thinly sliced (about 6); 2 bunches broccoli stems, peeled and thinly sliced (about 3 cups; see Cook's Note); 1 1/2 cups heavy cream; 2 teaspoons chopped fresh thyme leaves; Pinch freshly grated nutmeg; Kosher salt and freshly ground black pepper; 2 cups grated Gruyere (about 8 ounces)