Preheat oven to 400°F / 200°C.
Spray a 12-space regular-sized muffin pan with nonstick cooking spray or brush it with cooking oil. Set aside.
Melt butter in the microwave or on the stovetop. Whisk in grated garlic. Set aside to cool slightly.
Use a mandolin or very sharp knife to thinly slice the sweet potatoes. You can peel them first if you like, though it's not necessary. Aim for slices that are 1/16-inch thickness.
Place sweet potato slices in a large mixing bowl and pour butter over top. Add thyme leaves, salt, and black pepper.
Gently stir everything together, making sure that a small amount of butter gets between most of the slices.
Stack the sweet potato slices up in the prepared muffin tin, filling each section all the way up to the top or slightly higher.
Bake, uncovered, until sweet potatoes are golden brown on top and very tender, 30 to 40 minutes (this will vary depending on the thickness of the sweet potato slices).
Remove the muffin pan from the oven. If any of the sweet potato stacks have tipped over, use a spoon to gently nudge them back into place. Allow stacks to cool in the pan for 5 minutes before removing.