Heat the 1 Tbsp. oil in medium pot set over medium heat. Add the onion and garlic and cook 4 minutes. Mix in the quinoa, oregano and spices and cook and stir 1 minute more. Pour in the stock and bring to a boil. Cover the pot, lower the heat to medium-low, and cook, undisturbed, 15 minutes, or until the quinoa is tender and has absorbed the liquid.
Spoon the quinoa into a large bowl and cool to room temperature. Add the remaining ingredients, except the bell pepper, and toss to combine.
Preheat the oven to 375 degrees F. Line a baking sheet with parchment paper. Cut each pepper, lengthwise, in half, cutting through the stem end. Remove the seeds and membranes from inside of each half pepper. Spoon, pack and mound the quinoa mixture into the peppers. Set stuffed peppers on the baking sheet. Cover and bake 30 minutes. Uncover and bake 10 minutes more, or until peppers are just tender and filling is heated through.