Heat a large nonstick skillet over medium heat. Add the mushrooms to the dry pan, season with salt, and cook for 5 minutes or until some of the moisture cooks out and the mushrooms begin to caramelize. Add 2 teaspoons of oil along with the onion, season with salt and pepper, and cook until golden brown, 4 to 5 minutes. Stir in the garlic, cumin, and paprika, and cook for 1 to 2 minutes. Transfer to the bowl of a food processor along with the chipotles, to taste.

Place the skillet back over medium heat. Add the walnuts to toast for 1 to 2 minutes until nutty smelling, stirring frequently. Transfer to the food processor and pulse with the mushroom mixture until finely chopped. Scrape into a large bowl and set aside.

Add the black beans and rice to the food processor and pulse two to four times to coarsely chop and combine. Add to the mushroom mixture along with the egg, mustard, 1 tablespoon of breadcrumbs, 1/2 teaspoon of salt, and 1/4 teaspoon of pepper. Stir well to evenly combine. If the mixture is a little loose or too sticky to handle, add the other tablespoon of breadcrumbs.

Place the nonstick skillet over medium-high heat and add the remaining 2 teaspoons of oil. Divide the mixture into six equal pieces, about 1/2 cup each, and shape into burgers roughly the size of your buns Flatter patties will be generally firmer and crispier. Thicker patties will have a nice crisp exterior with a more tender center.

Cook until golden brown, 3 to 4 minutes per side. If making cheese areers, add a slice of cheese to each paste cover the pan with lith and allow the cheese to melt for about 1 minute. Serve the patties on toasted buns with your favorite toppings.