First, toast the finely-chopped walnuts in a small saute pan until a little darkened and fragrant. Set aside.
In medium, heavy-bottomed pot, heat 2 tablespoons of olive oil over medium heat. When hot, add the onions and green bell pepper and saute 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.
Stir in the chopped mushrooms and add the chile powder, dry mustard, and cumin (and garlic powder, if using), as well as some salt and pepper. Saute until the mushrooms have softened and reduced in size, about 5 to 7 minutes.
Add the tomato paste and stir well, coating all of the other ingredients. Add the crushed tomatoes, brown sugar, and worcestershire sauce.
Reduce to preferred sauciness—I prefer something that’ll stay put on a bun but isn’t too thick—and then 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!