Preheat oven to 350°. Lightly grease baking dish.
Melt butter in a skillet over medium. Once warmed, add the smashed garlic and saute for a minute or two, until it becomes nicely fragrant. Remove and discard garlic.
In a large bowl, place sweet potato slices, thyme leaves, salt & pepper. Pour garlic butter and 1 tbsp of olive oil over the potatoes, and mix so they are evenly coated.
Stack the slices on their edges in spiral around the baking dish. Brush olive oil over the tops. Sprinkle more thyme if desired.
Cover with foil and bake for 50 - 60 minutes. Increase oven temp to 425°, remove foil, brush top edges with olive oil, and continue to bake another 30-40min. Let it rest a few minutes before serving.