Preheat the oven to 375 degrees. Cut the tops off the bell peppers and remove the seeds and membranes. Place the peppers cut side up in a baking dish.
Heat the olive oil in a large skillet over medium heat. Add the ground turkey sausage and cook until browned, breaking it up with a spoon as it cooks.
Add the diced onion and minced garlic to the skillet. Cook for 3 minutes until the onion softens.
Stir in the diced apples and cook for another 2 minutes. Add the chopped spinach and cook until it wilts.
Mix in the cooked quinoa, dried sage, salt, and black pepper. Stir everything together and remove the skillet from the heat.
Spoon the filling evenly into the prepared bell peppers. Cover the baking dish with foil and bake for 25 minutes. Remove the foil and bake for 10 more minutes until the peppers are tender.