Rinse the quinoa under cold water in a fine mesh strainer.
In a medium saucepan, combine the quinoa and vegetable broth (or water). Bring to a boil, then reduce heat to low. Cover and simmer for about 15 minutes, or until the quinoa is cooked and the liquid is absorbed. Fluff with a fork when ready.
While the quinoa cooks, heat olive oil in a large skillet over medium heat. Add the diced onion and bell pepper. Sauté for about 5 minutes until softened.
Add minced garlic, cumin, chili powder, salt, and pepper to the skillet. Stir and cook for another 1-2 minutes until fragrant.
Stir in the black beans and corn, cooking for an additional 3-5 minutes to heat through. Remove from heat.
Combine the cooked quinoa with the vegetable mixture in the skillet. Mix well to combine all ingredients.
To assemble the tacos, spoon the quinoa and black bean mixture into taco shells or corn tortillas.
Garnish with fresh cilantro and serve with lime wedges and sliced avocado, if desired.