Heat oil in a large saucepan over medium heat. Add onion and cook 5 minutes, or until soft. Add garlic, chili powder, cumin, salt and pepper, and sauté 1 minute.
Stir in 3 cups cooked quinoa and the drained black beans. Cook for about 5 minutes, then turn down to low to keep hot.
Prepare the “Greek salsa” by combining tomatoes, cucumbers, onions in a bowl with fresh cilantro. Season with salt to taste; add in the fresh cilantro.
To make the sauce, place the yogurt, avocados, garlic, cilantro, lime juice, and cumin in a food processor. Mix until smooth. Salt and pepper, to taste.
Heat the corn tortillas.
On each tortilla, layer the quinoa and bean mixture with the Greek salsa, ending with the creamy avocado sauce on top.
Fold in half and serve. Serve with lime wedges if desired.