Preheat oven to 350 degrees.
Cut peppers lengthwise, preserving the stem end but carefully removing the core, seeds, and ribs.
Spray a 9 X 13 inch baking dish with cooking spray.
Using a large bowl, combine the ground beef, spinach, zucchini, onion, garlic, bulgur, egg, oregano, chili flakes, salt, and black pepper. Mix well with your hands, making sure to distribute vegetables and bulgur well.
Spoon mixture into pepper halves and arrange peppers with mixture-side up in baking dish.
Cut canned tomato pieces into smaller pieces by dipping kitchen shears into the cans and snipping away. Spoon 2-3 tablespoons of the canned tomato juices over the tops of the stuffed peppers. Spoon the tomatoes around the peppers, surrounding them on all sides.
Cover the baking dish with foil and bake for 30 minutes.
Remove baking dish from oven and carefully remove foil. Baste stuffed pepper tops with juices from baking dish. Sprinkle crumbled feta over tops of stuffed peppers.
Return to oven and bake uncovered for 45 minutes. The peppers will be tender and the cheese a lovely golden brown.