In a large bowl combine beef and pork, beaten egg, chopped mint and dried oregano, breadcrumbs, 3/4 of the feta cheese (reserve the rest for later) and salt, mix well and shape into large meatballs. You should get 20-21 meatballs.
Preheat the oven to 180C/350F. Heat the olive oil in a large pan, which could be used in the oven later, and brown the meatballs over high heat until golden but not cooked all the way through. (If you cook them till done, they will dry out in the oven later.) Remove to a separate plate.
To the same pan add the chopped onion, bay leaves, cinnamon sticks and cook over low heat for 5 minutes, then add pressed garlic, sliced red pepper and dried oregano and cook for 2-3 minutes longer. Then add in canned chopped tomatoes and the chicken stock with a pinch of salt, stir to combine and bring to a boil.
Once boiled add the orzo and stir, cover and cook in the preheated oven for 20 minutes. Then take out of the oven, uncover and top with the browned meatballs and cook for 10 minutes longer uncovered. Then scatter the remaining feta cheese all over and put in the oven for additional 7-10 minutes.