Preheat oven to 400°F (200°C). Wash and thinly slice squash (skins on). Using a mandolin may help with getting even baking of the squash pieces since your slices will be uniform.
Place cooling racks on cookie sheets (you will most likely need two so that the slices have room and aren’t overlapping too much) and line the slices on the racks.
You can also use parchment lined cookie sheets instead of the racks. Sprinkle with salt, and place 4 sprigs of thyme on each, spreading them out. Bake for 20-25 minutes, depending on thickness, until golden brown and crispy.
Kabocha squash releases water when cooked, especially if freshly picked, so cooking on cooling racks on top of a cookie sheet allows the pieces to get crispier.
Cooking on parchment paper on a cookie sheet works too. If using oil, use a spray so that there is not a solid layer of oil. This allows the water to evaporate instead of getting trapped inside and creating a mushy texture.