Set up a bowl of cold salted water. Peel the potatoes and as you peel them, put them in the bowl of salted water. Drain the potatoes, grate them and place them in a sieve set over another bowl. Let them drain them over the bowl collecting all of that liquid for a good 20 to 30 minutes. Press on the potatoes to get out more liquid.
Transfer the grated potatoes to a bowl. Slowly pour the water out of the bowl that was under the strainer until you can see the potato starch that’s collected in the bottom. Save the starch and add it to the grated potatoes. Grate the onion into the bowl of potatoes and season the mixture with salt, pepper and thyme.
Preheat the oven to 275°F. Place a wire cooling rack in a baking sheet.