Cut a side off each pepper to make large boats, peppers will be stuffed and remain on their sides. Remove seeds and ribs from peppers then finely chop the trimmed pieces.
Bring a pot of water to a boil. Add salt and boil peppers for 4 minutes then cold-shock in an ice bath. Drain and pat dry.
In a saucepot over medium-high heat, melt butter. Add pasta or orzo and toast. Add rice and season with salt and pepper. Add stock, bring to boil and cover. Cook 16 minutes then pour out onto a small tray to cool.
Heat a large skillet over medium-high heat with 2 tablespoons olive oil. Add chopped pepper trimmings, celery, half the onions, 4 cloves garlic, parsley, thyme, salt and pepper. Stir to soften 7-8 minutes; remove from heat to cool.
Heat a Dutch oven or large saucepot over medium heat with remaining olive oil, 2 turns of the pan. Add remaining onions and garlic, and bay leaf; season with salt and pepper. Soften a few minutes then add stock and cook to reduce by half. Add passata or purée and heat through, adjust seasonings to taste.
In a large bowl, combine meats and season with salt and pepper. Make a well in the center of the meat and add egg, cooled vegetables, rice and half the cheese. Combine the stuffing with your hands and fill the peppers.
Pour half the sauce into a large casserole dish then arrange the peppers in the dish. Top peppers with a little more sauce and remaining cheese. Roast peppers 30 minutes covered with foil and 15 more to brown and until a thermometer inserted into the middle reads 155°F.