Place mushrooms in the bowl of a food processor and process them in batches until they are finely chopped. Set aside.
In a large skillet (I used a cast iron skillet) heat 1 tablespoon of the olive oil. Add the onion and pepper. Cook, stirring frequently, until they are wilted, 5-7 minutes.
Stir in the tomato paste, salt, pepper, and chili powder. Cook, stirring constantly, until they are all incorporated, 3-4 minutes.
Add the mushrooms and cook until they release some of their juices, 5 minutes.
Stir in the beer, ketchup, and walnuts. Cook stirring occasionally for -7-10 minutes.
In the mean time, heat 1tablespoon olive oil in a large skillet. Add the red cabbage and cook, stirring occasionally, until soften, 7-8 minutes. Once cooked, place it in a mixing bowl. Stir in the balsamic vinegar. Season it with 1/4 teaspoon salt.
To assemble: Spoon 2 tablespoons (or more) of the vegan Sloppy Joe’s onto each bun. Top it off with wilted red cabbage and sliced avocados.