Prepare quinoa according to package directions; set aside.
Preheat oven to 375 degrees.
Heat the olive oil in large skillet over medium-high heat until fragrant. Add onion and garlic; cook for 2 to 3 minutes.
Add ground beef; cook, breaking up with a wooden spoon, until no longer pink. Add the diced tomato; cook until some of the liquid has evaporated. Sprinkle mixture with oregano, lemon zest, salt and pepper; remove from heat and set aside.
In a large bowl, combine cooked quinoa, meat mixture, olives, Feta cheese and 3 tablespoons of tomato puree; blend well.
Fill each bell pepper half with meat/quinoa mixture then place them in a 9- x 13-inch baking dish; carefully pour remaining tomato puree around them. Cover baking dish tightly with foil and bake for 40 minutes or until bell peppers are tender.