Cut the tops off the bell peppers and remove seeds and membranes. Set aside.
In a large skillet, heat olive oil over medium heat. Add chopped onion and cook until translucent, about 5 minutes.
Add ground beef to the skillet and cook until browned, breaking it up with a spoon.
Stir in rinsed rice, diced tomatoes, oregano, parsley, salt, and pepper. Add 1 cup of water, bring to a boil, then simmer until rice is partially cooked, about 10 minutes.
Stuff each pepper with the beef and rice mixture, then place in a baking dish.
Top each pepper with crumbled feta cheese.
Cover with foil and bake for 30 minutes. Remove foil and bake an additional 10 minutes until peppers are tender and cheese is slightly browned.
Notes
This classic Greek dish makes a perfect weeknight dinner and tastes even better the next day.