1 3/4 cups light beer (drink what's left in the bottle)
1/3 cup finely chopped walnuts
1 small green bell pepper, seeded and diced
Freshly ground black pepper
1/2 teaspoon chipotle chile 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
In a food processor, pulse the mushrooms in batches until very finely chopped. Place aside.
In a sizable nonstick skillet, heat the oil over medium-high heat. One tablespoon of beer, one-fourth of a teaspoon of salt, and the onions should be added at this point. Cook for about five minutes, stirring often. Cook, turning periodically, the peppers and walnuts for about 3 minutes, or until the peppers are crisp-tender. Cook the mushrooms, turning regularly, for about 5 minutes, until they are barely cooked through. Add the mushrooms, 3/4 teaspoon black pepper, and chipotle powder.
When the sauce reaches the desired consistency, about 2 minutes later, add the remaining beer, the ketchup, the tomato paste, and 1/8 teaspoon salt.
Distribute the mixture among the buns. Adding toppings is optional.