In a medium pot, heat the olive oil over medium heat. Add the onion and garlic and sauté for about 4 minutes, or until the onions turn translucent.
Add the water, quinoa, cumin, and red pepper flakes.
Bring the water to a boil, then reduce the heat, cover, and simmer for 12 to 15 minutes, or until the quinoa is fully cooked and fluffy.
Stir in the black beans, lime juice, salt, and pepper.
Spoon the quinoa mixture into the warm corn tortillas, top with the avocado slices, and serve.