Bring water and quinoa to a boil in a saucepan. Reduce heat to medium-low, cover, and simmer until quinoa is tender, 15 to 20 minutes
Heat olive oil in a large skillet over medium heat. Add onion and cook until softened, about 5 minutes. Add turkey and season with salt and pepper; cook and stir until turkey is no longer pink, 5 to 7 minutes. Add mushrooms and cook until softened, about 5 minutes
Preheat the oven to 350 degrees F (175 degrees C)
Transfer cooked turkey mixture to a large bowl. Add cooked quinoa, tomato sauce, and tomato paste. Stir well; add additional tomato sauce as needed, until filling has the consistency of a casserole
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