For the meatballs, Heat 1 tablespoon of olive oil in a large saute pan and add the onion. Fry gently for 5 minutes until soft, then add the garlic and thyme and cook for 2 minutes. Let cool.
Put the beef in a large bowl and add the cooled onion mix. Add 3 tablespoons of breadcrumbs and mix well. The amount of crumbs required depends on how much water is in the meat. If the mixture is not lumping together and feels wet, add a little more. Season well with salt and pepper. Rooll the mixture into balls a little larger than a walnut. (You should get about 16).
Heat the reamining 2 tablespoons of olive oil in the saute pan and fry half of the meatballs, turning regularly, until they are browned all over. Remove them from the pan with tongs or a slotted spoon and set aside, then fry the remaining meatballs and once browned, set those aside too. Wipe out the Pan.
For the peperonata, add the oil to the pan and fry the onion for 8 minutes until softened. Add the garlic and fry for a couple more minutes, then add the peppers and tomatoes. Add a splash of water to the can, swirl it around and tip that in too. Increase the heat to a simmer and cook the sauce for 30 minutes, uncovered, until it has reduced down and thickened. Meanwhile, bring 4 cups of water to boil in a saucepan.
Once the 30 minutes are up, add the vinegar to the sauce and season well with salt and pepper. Return the meatballs to the pan and tuck them into the sauce. Put the lid on the pan and cook for another 10 minutes, until the meatballs are cooked through.
For the capers, add a drizzle of oil to a frying pan over a high heat and then add the capers. Fry for about 5 minutes until crisp.
Add the polenta to the boiling water in the pan, pouring it in a slow stream while beating with a spoon. Keep beating for about 2 minutes, until the polenta is thickened and no longer gritty. Season well.
To serve, divide the polenta between four plates and top with meatballs and sauce. Sprinkle with capers and chopped parsley.