Heat a skillet over medium heat. Once hot, add walnuts and toast them for about 8 minutes, stirring them occasionally until they start to turn golden brown. Cool completely.
Grind the walnuts in the food processor just until they resemble breadcrumbs. Set aside.
In the same skillet, heat olive oil over medium heat. Once hot, add the onions, and sauté until for 3 minutes. Add the garlic and sauté for an additional 2 minutes.
Add mushrooms and salt to the skillet.
When the mushrooms start releasing liquid, add the cumin, chili powder, smoked paprika, Italian seasoning, and black pepper. Cook until all the liquid evaporates.
In a large bowl, mix together the black beans, brown rice, sautéed mushrooms mixture, ground walnuts, breadcrumbs, and barbecue sauce. Mix until everything is well incorporated. Adjust the seasoning as desired.
Divide the burger mixture into 5 firmly packed patties, approximately ¾ inches thick. .
To grill the burgers, preheat your grill over medium heat. Cook for about 5 minutes, then flip gently. Cook for an additional 4-5 minutes on the other side.
To bake the burgers, bake on a well-greased baking sheet at 375°F for 30-40 minutes, flipping halfway through.
To fry the burgers, heat a skillet over medium-high heat. Add olive oil to coat the pan. Cook burgers on one side for 5 minutes, then flip. Cook for an additional 4-5 minutes until golden brown.
Can freeze raw burgers by placing them on a baking sheet lined with parchment paper. Once semi-frozen, stack burgers with parchment paper in between each burger. Store in air-tight container for up to 6 months.