Preheat oven to 400 degrees Fahrenheit. Coat a baking pan with two tablespoons melted butter.
Wash potatoes. Using a sharp knife, cut off and discard the ends. Slice them into thin rounds, no thicker than ¼-inch. (read the recipe notes for tips).
Carefully arrange the slices upright in a greased pan - tuck in garlic and thyme among slices.
Brush slices with the remaining two tablespoons melted butter. Season with salt and pepper to taste.
Bake for 45-60 minutes or until potatoes are tender and crisp. Garnish with fresh thyme.
Make sure to get potatoes uniformed in size.
Sweet potatoes are tough to cut, so I like to microwave them for a few minutes to soften them up a bit. If you want to omit this step, make sure to sharpen your knife! You can also use a mandoline slicer, but I find these slices are too thin. It is excellent for making sweet potato chips, though.
If your sweet potatoes are small, you can make them Hasselback-style: cut them almost all the way through (left joined at the bottom) into thin, fan-like slices. Just make sure to serve one potato per person.
You can use a 9-inch square or round metal or ceramic baking pan. I wouldn't suggest using a glass pan. I find that potatoes don't get as crispy in it.
Different types cook differently, so it is imperative to check them during cooking than relying solely on time. Not to mention that oven temperatures vary too!