Preheat oven to 325°F.
For the filling, heat a medium saucepan over medium heat with EVOO, 2 turns of pan. Add celery, 1 onion, cumin and caraway seeds, 2 cloves garlic and season with salt and pepper. Cook to soft and tender, stirring frequently, about 8-10 minutes. Let cool.
For the sauce, heat a medium saucepot or Dutch oven over medium heat with EVOO, 2 turns of the pan. Melt butter into oil, add remaining onions, garlic and the bay leaves; season with salt and pepper. Cook to soften, 7-8 minutes, then stir in flour. Cook 1 minute then whisk in 2 1/4 cups chicken stock. Cook a minute to thicken then add passata and lower heat to a simmer.
Heat another saucepot over medium-high heat with EVOO, 1 turn of the pan. Add rice and season with salt and pepper. Toast grains 2 minutes then add 1 3/4 cup chicken stock and bring to boil. Cover pan and reduce heat to simmer. Cook 18 minutes to fluffy and just tender. Spread out on a small sheet tray to cool.
Place the ground meats in a large bowl and season with salt, pepper, paprika and half the parsley. Add rice, egg, cooled vegetables and mix well to combine.
Cut the tops off of the peppers and remove seeds and ribs. Fill peppers with meat mix to the top; roll the remainder of meat mix into 2 1/2-inch meatballs.
Ladle half the sauce into the bottom of a large casserole dish and arrange peppers and meatballs in the pan. Cover with remaining sauce and roast 90 minutes to 1 hour 45 minutes, basting occasionally and adding a little stock or water if sauce falls below mid height of peppers.
Arrange peppers and meatballs on platter and cover to keep warm. Transfer sauce to a skillet and whisk over medium-high heat to thicken and smooth. Remove bay leaves.
Serve peppers and meatballs on dinner plates topped with more sauce and garnished with remaining parsley and chives. Pass sour cream at table for topping.