1. First, toast the finely-chopped walnuts in a small sauté pan until a little darkened and fragrant. Set aside.
2. In medium, heavy-bottomed pot, heat 2 tablespoons of olive oil over medium heat. When hot, add the onions and green bell pepper and sauté until the onions are translucent, 6 to 8 minutes. Then add the serrano pepper and garlic and cook, stirring, until the garlic is golden, 3 to 4 minutes.
3. Stir in the chopped mushrooms and add the chili powder, dry mustard, and cumin (and garlic powder, if using), as well as some salt and pepper. Sauté until the mushrooms have softened and reduced in size, about 5 to 7 minutes.
4. Add the tomato paste and stir well, coating all other ingredients. Add the crushed tomatoes, brown sugar, and Worcestershire sauce.
5. Reduce to preferred sauciness and stir in the toasted walnuts. Season with salt and pepper to your liking. Warm a few potato buns, and spoon the joe mix onto it. Garnish with pickled red onions and cilantro. Eat it open faced or as a sandwich!