Preheat oven to 350 degrees. Line a 13x9 baking dish with parchment paper or foil. Slice the tops off the peppers and remove the ribs and core.
In a large bowl, add the ground meat, spinach, zucchini, onion, bulghur, egg, pesto, oregano, and salt and pepper. Mix well.
Place peppers inside the baking dish. To ensure they remain standing, you can make barriers out of foil to place between each pepper. Fill the peppers with the meat mixture.
Pour the stewed tomatoes over the peppers and sprinkle generously with crumbled feta.
Cover with foil and bake for 30 minutes.
Remove the cover and bake for another 30 minutes, until the meat mixture is thoroughly cooked and the peppers have softened.
Serve warm and pair with rice or vegetables.