I don't remember where I found this but it had to be back in 1999.........I was going through a Vegan Stage. I do know I tweaked it as I have ingredients crossed out & added by me. I hope my Vegan friends are kind when they critique this Vegan recipe by a non vegan.
Pulse mushrooms in batches in a food processor until finely chopped. Set aside.
Heat oil in a large skillet over medium-high heat. Add onions, 1 tablespoon beer and 1/4 teaspoon salt and cook, stirring frequently, until onions are lightly browned, about 5 minutes.
Add walnuts and peppers and cook, stirring occasionally, until the peppers are crisp-tender, about 3 minutes. Add mushrooms, 3/4 teaspoon black pepper,chipotle powder, dry mustard, garlic powder and cook, stirring frequently, until the mushrooms are just cooked through, about 5 minutes
Add remaining beer, ketchup, tomato paste, and 1/8 teaspoon salt and cook while stirring until the sauce is the consistency that you like, about 2 minutes. Spoon mixture onto each bun.