Pulse the mushrooms in batches in a food processor until finely chopped. Set aside.
Heat the oil in a large nonstick skillet over medium-high heat. Add the onions, 1 tablespoon beer and 1/4 teaspoon salt and cook, stirring frequently, until the onions are lightly browned, about 5 minutes. Add the walnuts and peppers and cook, stirring occasionally, until the peppers are crisp-tender, about 3 minutes. Add the mushrooms, 3/4 teaspoon black pepper and chipotle powder and cook, stirring frequently, until the mushrooms are just cooked through, about 5 minutes.
Add the remaining beer, the 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 the mixture onto each bun. Serve with toppings if desired.
Per serving (not including optional ingredients): Calories: 257; Total Fat 9 grams; Saturated Fat: 1 gram; Protein: 8 grams; Total carbohydrates: 37 grams; Sugar: 12 grams Fiber: 5.5 grams; Cholesterol: 0 milligrams; Sodium: 486 milligrams