Heat the olive oil over medium-high heat in a large frying pan until shimmering. Add onion, garlic, and celery and season with salt and black pepper. Cook, stirring occasionally, until soft and starting to brown. Add beef and cook until browned, breaking meat up with the back of a spoon. Season again with salt and pepper.
Add tomatoes with it’s juice, mint, oregano, and cinnamon and simmer for about 5 minutes. Taste and add more salt and pepper if necessary. Add rice and simmer just until the liquid has been absorbed. Remove cinnamon stick.
Heat oven to 350°F.
Slice off enough of the stem end of each pepper to expose the cavity; reserve the tops. Remove all the seeds then loosely fill each cavity with the meat filling, leaving enough room for the filling to expand. Replace the tops and place the peppers in a 9-x-13-inch baking dish and pour in enough water to reach 1/2-inch up the sides of the peppers.
Bake, basting every 15 minutes, until rice is tender and peppers have browned, about 1 hour 15 minutes.