Prepare the quinoa.
While the quinoa is cooking, whisk the olive oil, cumin, garlic, lime, salt, and cayenne (if using) together in the bottom of a large bowl to let the flavors marry while you chop the veggies.
Rinse and drain the black beans, then chop the veggies; the key is to make the veggies about the same size as the beans.
Add the cooked quinoa, beans and veggies to the bowl and gently fold it all together with the dressing.
The quinoa can be warm, room temp, or cold when you make the dish. Regardless, give it a while to sit in the fridge to let the flavors meld a bit.