Cook the quinoa in vegetable broth according to the package directions, about 15 minutes. Once it's fluffy and tender, fluff with a fork and set aside while you prepare the remaining ingredients.
Heat the olive oil in a skillet over medium heat. Add the onion and cook for 4-5 minutes, until softened. Stir in the garlic and cook for 30 seconds, until fragrant. Add the black beans, taco seasoning, and salt, stirring to coat. Cook for 3-5 minutes, until the beans are heated through. Remove from the heat and set aside.
To make the dressing, add all ingredients to a blender or food processor and blend until smooth. If needed, add 2-4 tablespoons of water, one tablespoon at a time, until the dressing reaches your desired consistency. Taste and adjust as needed, then transfer to a jar or container.
Prep the remaining ingredients by chopping the bell pepper, halving the cherry tomatoes, chopping the lettuce, and slicing the avocado.
Assemble the bowls by layering quinoa, lettuce, bean mixture, bell pepper, tomatoes, and cilantro. Toss lightly if desired. Drizzle with dressing and top with avocado just before serving.