Heat the oven to 425°F. Combine the rinsed quinoa and 1 1/2 cups water in a small saucepan; bring to a boil, cover, reduce the heat to low, and simmer 15 minutes. Turn off the heat and let stand, covered, 5 minutes, then fluff with a fork.
Halve the bell peppers lengthwise through the stem and scoop out the seeds and ribs. Arrange them cut-side up in a 9x13-inch baking dish.
Heat the olive oil in a large skillet over medium-high. Add the onion and cook 4 minutes, until soft. Add the garlic and cook 30 seconds. Add the turkey, oregano, salt, and pepper; cook 6 to 8 minutes, breaking up the meat, until no longer pink and the thickest part reaches 165°F.
Stir the marinara and cooked quinoa into the skillet and simmer 2 minutes. Spoon the filling into the pepper halves, mounding it slightly.
Bake uncovered 22 minutes.
Scatter the mozzarella over the peppers and bake 8 to 10 minutes more, until the cheese is melted and the peppers are tender. Let rest 5 minutes before serving.
