Cook the quinoa according to the package directions.
Preheat oven to 375 degrees F and lightly grease a 9x13 baking dish or rimmed baking sheet. Brush the halved peppers with olive oil and place them cut side up on the baking sheet
Heat the olive oil in a large skillet over medium high heat. Add the onion and sauté for 5 minutes. Add the tuna and stir to combine. Add the cumin, chili powder and garlic powder and incorporate. Add the tuna mixture to the quinoa. Stir in the black beans, cheese, salsa and green onions. Stir to combine. Taste and adjust salt and pepper as needed.
Generously stuff halved peppers with the tuna quinoa mixture until all peppers are full, then cover the dish with foil.
Bake for 30 minutes covered, then remove foil, sprinkle a little extra cheese on top, increase heat to 400 degrees F, and bake for another 15 minutes, or until peppers are soft and slightly golden brown
Garnish with assorted toppings listed above.