Step 1: Preheat your oven to 375°F (190°C) and generously grease a 9x13 inch baking dish with butter.
Step 2: In a small saucepan over medium-low heat, combine the heavy cream, smashed garlic, thyme, and nutmeg. Simmer gently for 5 minutes, then remove from heat and let steep.
Step 3: Prepare the kale by removing the tough ribs and chopping the leaves into small pieces. Season lightly with salt.
Step 4: Arrange a layer of sweet potato slices in the bottom of the prepared dish, overlapping them slightly.
Step 5: Scatter a third of the chopped kale and a third of the shredded Gruyère cheese over the potato layer. Season with a pinch of salt and pepper.
Step 6: Repeat the layering process twice more, ending with a final layer of sweet potatoes and cheese on top.
Step 7: Remove the garlic cloves from the cream and pour the liquid evenly over the entire dish.
Step 8: Cover the dish tightly with aluminum foil and bake for 40 minutes.
Step 9: Remove the foil and bake for an additional 20 minutes, or until the top is golden brown and the potatoes are tender when pierced with a fork.
Step 10: Let the gratin rest for 10-15 minutes before slicing to allow the sauce to set.