In a large skillet, heat ½ teaspoon of olive oil over medium heat. Add the onion and cook until soft, about 2 minutes.

Add the mushrooms, pinches of salt and pepper, and cook until browned and soft, 5 to 8 minutes.

Add ½ cup of the black beans, garlic, the balsamic vinegar and tamari sauce, and cook for 2 minutes or until everything is well browned and soft.

Remove the mixture from the heat and let cool slightly.

In a food processor, pulse the walnuts. Add the mushroom mixture and pulse 2 to 3 times until just combined. Do not puree or the mixture will be too mushy.

Add the remaining ½ cup of black beans and pulse briefly.

Transfer the mixture to a large bowl and stir in the brown rice, chili powder, onion powder, salt and pepper. Mix until combined. Taste and adjust seasonings.

Preheat the oven to 400°F and line a large baking sheet with parchment paper.

Spread the panko breadcrumbs onto a plate. Form the mixture into 18-1½ inch balls and gently roll in the crumbs, using the breadcrumbs to pat the mixture into cohesive balls.

Place on the baking sheet and refrigerate for 20 minutes or overnight.

Drizzle generously with olive oil and bake for 35 to 40 minutes, carefully flipping halfway through.

Heat the marinara sauce in a skillet and add the meatballs. Heat until just warmed through and serve immediately on the buns with arugula, basil, and parmesan cheese, if desired. (Tip: don’t let the meatballs sit in the sauce for too long or they will start to fall apart).