For the filling, heat olive oil, 2 turns of the pan, in a large skillet over medium-high heat. Add the pancetta and stir a minute or two; add beef and brown and crumble. Add onions, garlic and chili pepper flakes, and season with salt and pepper. Cook 5 minutes, stirring occasionally. Add tomato paste and stir 1 minute; add wine and stir. Add stock, tomato purée or passata and milk, and reduce heat; stir in basil. Simmer to thicken and combine flavors, about 15-20 minutes.
To serve, make garlic bread rolls for your sloppy beef: Heat oil in a small pan over medium heat. Melt butter into oil. When it foams, add garlic and swirl 1-2 minutes; remove from heat. Stir parsley into butter. Lightly toast rolls in a hot oven or under the broiler. Brush rolls with garlic butter and sprinkle cheese over rolls, return to oven 1 minute or place under broiler for 30 seconds or so to set cheese.