Prepare the Ingredients

1. Preheat Oven: Preheat your oven to 375°F (190°C).
2. Slice Sweet Potatoes: Use a mandoline or sharp knife to slice the sweet potatoes into thin rounds, about 1/8 inch thick.

Make the Cream Mixture

1. Combine Cream and Garlic: In a medium bowl, mix together the heavy cream, minced garlic, thyme, salt, and pepper.
2. Add Cheeses: Stir in the shredded Gruyère cheese and grated Parmesan cheese until well combined.

Assemble the Gratins

1. Layer the Sweet Potatoes: In a greased muffin tin or individual ramekins, layer the sweet potato slices, slightly overlapping each slice.
2. Pour Cream Mixture: Pour the cream and cheese mixture over the sweet potatoes until just covered.

Prepare the Topping

1. Mix Panko Topping: In a small bowl, combine the panko breadcrumbs, melted butter, and additional Gruyère cheese.
2. Sprinkle on Top: Evenly sprinkle the panko mixture over each gratin.

Bake the Gratins

1. Bake: Place the muffin tin or ramekins in the preheated oven and bake for 30-35 minutes, or until the sweet potatoes are tender and the tops are golden brown.
2. Cool Slightly: Allow the mini gratins to cool for a few minutes before serving.