Lightly oil the insert of a 6- to 7-quart slow cooker and spread the onion evenly in the bottom.
In a large bowl, combine the flour, yeast, onion powder, 1/2 tsp. of the thyme, 1/2 tsp. of the salt, and 1/4 tsp. of pepper. Add the water, soy sauce, and ketchup. Mix well, adding a little more water if the mixture is too dry, then knead for 2 minutes, until smooth. Shape the gluten to fit inside your cooker and place it on top of the onions.
Arrange the carrots and potatoes in the cooker around the seitan. Season the vegetables with salt and pepper to taste, and add the broth, garlic and remaining 1/2 tsp. of the thyme. Put the lid on the slow cooker and cook on Low until the seitan and vegetables are cooked, about 8 hours.
To serve, remove the vegetables and seitan from the slow cooker. Cut the seitan into slices and arrange them on a serving platter. Surround with the vegetables and spoon the cooking liquid over all. Serve hot.
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