Heat the vegetable oil in a large soup pot over medium heat. Add the onion and garlic and saute for 4-5 minutes. Add the curry paste and stir to coat the onions and garlic. Add the coconut milk and vegetable stock and bring to a boil.
In the meantime, peel the carrots and chop into 1/2-inch pieces. Peel the potatoes (or don't, if you're using potatoes with thin skin, like Yukon Golds) and chop into 1-inch pieces. Add to the pot along with the ginger, cumin, coriander, and cardamom. Once the curry starts to boil, turn down to a simmer and let cook for 10 minutes.
Break the seitan into bite-sized pieces, add to curry and cook for 15 more minutes, or until the vegetables and potato