Add the chickpeas, breadcrumbs, ¼ cup red onion, garlic, oil, egg, 2 tablespoons soy sauce, tomato paste, ½ teaspoon salt, cumin, and pepper to a large food processor. Pulse the mixture to break down the beans but keep the mixture chunky.
Form the mixture into 18 balls, about 1 ½-inches in diameter, and place them on a baking sheet. Spray the tops with cooking spray and bake for 20 minutes, turning and re-spraying halfway.
Add the butter to a large frying pan and bring the heat to medium-high. Add the remaining ¼ cup red onion and sauté for 3 minutes. Add the mushrooms and cook for 5 minutes. Add the flour and stir to coat, then add the broth, remaining soy sauce, miso paste, Worcestershire, thyme, garlic powder, and the remaining salt. Simmer for 5 minutes.
Top the meatballs with gravy and serve, optionally garnished with chopped chives.