Place the ground chicken into a large bowl. Add the black beans, tomatoes with their juices, quinoa, chili powder, cumin, garlic powder, salt, and 3/4 cup of the cheese. With a fork or your fingers, stir to combine. Spoon the filling into the hollowed-out centers of the peppers, filling them all the way to the top. (The number of peppers you need for all of the filling will vary depending upon the size of the peppers. I usually fill about 7.)
Pour 1/2 cup water into an empty 5-quart (or larger) slow cooker. Stand the peppers up on their bases in the water, arranging them so that they are evenly spread apart. Cover the slow cooker and cook on low for 6 hours or on high for 3 hours, until the peppers are tender. Remove the lid, sprinkle the peppers with the remaining 1/2 cup cheese, then cover the slow cooker again and let heat for a few additional minutes, until the cheese is melted. Serve hot with desired toppings.
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