Step 1: Prepare the Sweet Potatoes

1. Preheat your oven to 375°F (190°C).
2. Peel the sweet potatoes and slice them thinly (about ⅛-inch thick). Use a mandoline for even slices if possible.
3. Toss the sweet potato slices with olive oil, minced garlic, salt, and pepper in a large mixing bowl.

Step 2: Make the Cream Mixture

1. In a medium saucepan, combine the heavy cream and milk.
2. Heat the mixture over medium heat until it begins to simmer. Remove from heat and set aside.

Step 3: Assemble the Gratin

1. Lightly grease a 9×13-inch baking dish with butter or non-stick spray.
2. Arrange a layer of sweet potato slices at the bottom of the dish, overlapping slightly.
3. Pour a bit of the cream mixture over the sweet potatoes and sprinkle with grated Parmesan cheese.
4. Repeat the layers until all the sweet potato slices and cream mixture are used up. The top layer should be coated with Parmesan cheese.

Step 4: Bake the Gratin

1. Cover the dish with aluminum foil and bake in the preheated oven for 45 minutes.
2. Remove the foil and continue baking for an additional 15-20 minutes, or until the sweet potatoes are tender and the top is golden brown.

Step 5: Prepare the Rosemary Crumble

1. While the gratin is baking, prepare the crumble. In a small bowl, mix the panko breadcrumbs, melted butter, chopped rosemary, and toasted pine nuts.
2. Season with salt and pepper, and stir until the mixture is well combined.

Step 6: Add Burrata and Crumble Topping

1. Remove the gratin from the oven and tear the burrata cheese over the top, distributing it evenly.
2. Sprinkle the rosemary crumble mixture over the burrata.
3. Return the dish to the oven and bake for an additional 10-12 minutes, or until the topping is crispy and golden brown.

Step 7: Serve

1. Allow the gratin to cool for 5-10 minutes before serving.
2. Garnish with fresh rosemary sprigs, extra toasted pine nuts, and a drizzle of olive oil.