1 pound cremini mushrooms, halved
1 tablespoon extra-virgin olive oil
1 large sweet onion, diced
1 3/4 cups light beer
1/3 cup finely chopped walnuts
1 small green bell pepper, seeded and diced
Freshly ground black pepper
1/2 teaspoon chipotle chili powder
1/4 cup ketchup
3 tablespoons tomato paste
6 whole grain hamburger buns
Shredded red cabbage or lettuce, pickled jalapeno and scallions, for serving, optional
Pulse mushrooms in batches in a food processor until finely chopped. Set aside.
Heat oil in a large nonstick 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, and chipotle 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. Serve with toppings if desired.