Preheat oven to 425°F.
Place beef and sausage in a bowl. Squeeze excess milk from bread and add to meat, along with salt, pepper, egg, cheese, parsley, grated garlic, a drizzle of good olive oil and about 3 tablespoons grated onion. Mix with your hands to combine. Roll large, walnut-sized meatballs and arrange on parchment-lined, rimmed baking sheet. Roast to lightly browned and just cooked through, about 18-20 minutes.
While the meatballs cook, heat 2 tablespoons olive oil in a large, deep skillet or medium pot over to medium-high heat. Add sliced garlic and grate in the rest of the onion. Season with salt and pepper, and add red pepper flakes and oregano. Stir 2-3 minutes, add tomato paste and stir a minute more. Add wine and reduce by half; add stock and passata or sauce and crushed, diced or hand-crushed tomatoes, reduce heat to low to simmer.
Slide cooked meatballs into sauce and keep warm over low heat or in slow cooker. Serve with bread and toppings for meatball sliders.