In a slow cooker or a dutch oven, combine potatoes, mushrooms, carrots, onions, garlic, thyme, wine, tomato paste and 2 1/2 cups of vegetable broth. Give it a gentle stir and close the lid.
Cook for 3-4 hours on medium heat (or high setting of the slow cooker) or 6-8 hours on low heat (or low setting).
In a separate bowl, whisk together remaining 1/2 cup of the veggie broth with cornstarch. Add to the roast mixture and give it another stir.
Cook for another 2-3 minutes as you let the mixture thicken up. Serve garnished with fresh parsley if desired.