Coat the bottom of a large pot with the oil and place it over medium heat. Once the oil is hot, add the onion, celery, and carrots. Sweat the vegetables for about 5 minutes, stirring occasionally, until they start to soften.
Add the seitan and cook the mixture for another 5 minutes, until the seitan just starts to brown at the edges, stirring occasionally.
Stir in the garlic and cook everything for about a minute longer, stirring constantly, until the garlic becomes very fragrant.
Stir in the red wine. Bring it to a simmer and let it cook for 3 to 4 minutes, until reduced by about half.
Stir in the broth, tomatoes (including any juice in the can), barley, thyme, bay leaves, and black pepper. Raise the heat and bring the broth to a simmer. Lower the heat and simmer the soup, uncovered, until the barley is tender, 40 to 50 minutes, stirring occasionally. You can add some water if the broth reduces too much.
Take the pot off the heat. Remove the bay leaves and season the soup with salt to taste, if desired. Stir in the parsley.
Ladle into bowls, serve, and enjoy!