Preheat your oven to 400 degrees with the tray in the middle rack. Preheat a medium size skillet over medium high heat.
Cut the zucchini's in half lengthwise, and scoop out the seeds with a melon baller or spoon. On a microwave safe plate or baking dish, microwave the scooped out zucchinis for 4 minutes over high heat, or until they are tender when pierced with a fork.
Chop the zucchini seeds and flesh you scooped out, and cook it in the pan with the onion and olive oil. Season with a pinch of salt and pepper. Cook until all the water has evaporated from the zucchini, and the onion is translucent. About 6 minutes. Add in the garlic and cook 30 more seconds, then transfer the cooked zucchini and onion to a medium size bowl.
In the same medium size mixing bowl, combine the cooked zucchini and onion, cooked quinoa, black beans, corn, 1 cup shredded cheese, and about half the can of enchilada sauce. Stir to combine. You want the quinoa mixture to be evenly coated in the enchilada sauce, but not dripping or overly saturated.
Transfer the microwaved zucchinis to an oven safe baking dish. Mine fit in a 9x13 baking dish, but you may need 2 depending on the size of your zucchinis. Season the microwaved zucchinis with a pinch of salt and pepper, then evenly distribute the quinoa mixture inside. Top with remaining 1 cup of shredded cheese. Bake uncovered for 10 minutes, or until the cheese has melted and the mixture is heated through. Serve warm with avocado slices and sour cream.