In a large pot, bring the chicken broth to a boil. Add the rinsed quinoa, reduce heat to low, and cover. Simmer until the quinoa is tender and fluffy, about 15 minutes.
While the quinoa cooks, heat olive oil in a saute pan over medium heat. Add the minced garlic and dried sage, stirring until fragrant—about 30 seconds—and then add the chicken chunks. Cook until the chicken is browned on all sides and cooked through, about 8-10 minutes.
Preheat your oven to 375°F (190°C). In a mixing bowl, combine the cooked quinoa, chicken, cranberries, and shredded cheese. Season with salt and pepper to taste, then mix gently to distribute everything evenly.
Transfer the mixture into a greased baking dish, spreading it out into an even layer. Sprinkle a little extra cheese on top for a golden finish.
Bake uncovered in the preheated oven until bubbling around the edges and the cheese on top is golden brown, about 20-25 minutes.
Remove from the oven and let the casserole rest for a few minutes. Serve warm, enjoying the savory chicken melded with the sweet cranberries and aromatic sage.