Preheat oven to 375°F. Grease a 9x13-inch baking dish.

Slice sweet potatoes and potatoes into thin, even rounds (about ⅛ inch thick). Set aside.

In a saucepan over medium heat, melt the butter. Add garlic and cook for 30 seconds, just until fragrant.

Stir in the cream, milk, salt, pepper, smoked paprika, and thyme. Heat until warm but not boiling, then remove from heat.

Layer half of the potatoes in the baking dish, alternating sweet potatoes and regular potatoes. Pour half of the cream mixture over the top and sprinkle with half of the cheddar cheese.

Repeat with remaining potatoes, cream mixture, and cheddar cheese. Finish with Parmesan on top.

Cover tightly with foil and bake for 40 minutes.

Remove foil and bake an additional 20 minutes, or until the potatoes are tender and the top is lightly browned.

Let rest for 10 minutes before serving. Garnish with fresh herbs if desired.