Clean and cut sweet potatoes to 1/2-inch thick rounds ( you can either keep the skin on or peel it).
Preheat oven to 200 Celcius / 400 F
To sliced sweet potatoes add melted butter, cinnamon(1 teaspoon), salt, and pepper. Using a spatula or both hands, mix everything well. Instead of a bowl, you can also use a ziplock bag, add everything to it and shake to mix.
Arrange sweet potato slices on a baking tray in a single layer leaving little space between to slices. (do not overcrowd the pan, else the potatoes will steam instead of browning) . Bake for 25-30 minutes @200 d Celcius /.
At the end of the baking time, the sweet potatoes should be form tender. Baking time may vary depending on the thickness of the slices.
Meanwhile, mix maple syrup, chopped walnuts, cinnamon, and thyme in a bowl.
Remove the baking tray. Top each sweet potatoes slices with about 1 teaspoon of the above mixture.
Put the tray back into the oven and bake for another 8-10 minutes or unitl the sweet potatoes looks caramelized around the edges.
Serve as it is or with some feta sprinkled on top.