Preheat the oven to 375 degrees F and line a 9 x 13 dish with aluminum foil.
Wash the bell peppers, cut of the tops, and remove the seeds from the inside. Place each bell pepper "standing up" in the dish. Bake the bell peppers for about 7-10 minutes until slightly softened, remove from oven and set aside.
Meanwhile, heat olive oil on medium heat and sauté the onion and garlic until translucent, about 2-4 minutes. Cook quinoa as directed on package (usually 1 cup quinoa:2 cups water ratio).
Add the ground turkey to the pan with garlic and onion and finely chop the ground turkey while cooking.
Once turkey is cooked through add the crushed tomatoes and reduce heat to medium low and simmer until quinoa is ready. Once cooked, stir quinoa into the turkey and crushed tomato mixture.
Optional: add a couple handfuls of spinach to the mixture and stir until wilted.
Fill the peppers by alternating a layer of cheese with a layer of the turkey/quinoa/sauce mixture until each pepper is completely full! Top with cheese and bake for about 20 minutes, until cheese is melted and brown on top and internal temp is at least 165 degrees F.
Remove from the oven, slice a pepper in half, and enjoy! Leftovers can be stored in an airtight container in the fridge for up to 4 days.