Cut the top off of each pepper with a knife a serrated knife.
Dice the pepper part of each pepper and transfer to a large bowl. Set aside.
Remove seeds from remaining peppers and rub outside of peppers with oil.
Place on a baking sheet and set aside while making the filling.
FILLING:
Pre-heat a large skillet over medium heat. Add remaining olive oil. Add garlic and cook for 2 to 3 minutes until fragrant.
Cook beef and onion over medium heat 8 to 10 minutes, stirring occasionally until beef is brown; drain.
Add tomato and cook one minute.
Add broth and bring to a boil.
Add quinoa, salt, and pepper.
Cover and simmer on low for 15 minutes or until liquid is almost absorbed. Remove from heat. Stir in half of the cheese. Set aside to cool for 10 minutes.
Stuff peppers with beef mixture. Stand peppers upright in ungreased 8-inch square glass baking dish.
Cover tightly with foil. Bake 10 minutes. Uncover and bake about 15 minutes longer or until peppers are tender.
Sprinkle with cheese. Wait until cheese is melted. Transfer to a platter and serve.