Preheat oven to 400ºF. Combine quinoa, vegetable broth, chili powder, cumin, and granulated garlic in a small saucepan. Bring mixture to boil, reduce heat, cover, and simmer until quinoa is fluffy, about 12 to 15 minutes. Transfer quinoa to a large bowl.
While the quinoa cooks, prepare the bell peppers. Arrange the bell peppers cut side up in a 13x9-inch baking dish with ~1/4 inch of water in the bottom. Cover with foil and bake at 400ºF for 15 to 20 minutes, until fork-tender.
To the bowl with quinoa, add black beans, salsa, mango, cilantro, lime juice, adobo sauce, and salt; mix well to combine. Taste the mixture and adjust seasonings as needed.
Divide the filling evenly into each bell pepper half. Replace foil and place back in the oven for 15 to 20 minutes. Garnish with avocado slices and extra lime juice, if desired. Add extra toppings of choice (Greek yogurt/sour cream or coconut yogurt, extra salsa, nutritional yeast or cheese/queso).