Heat the oil in a frying pan set over a high heat. Add the onion and garlic and cook, stirring, for 2–3 minutes, until softened. Add the tomatoes, tomato purée, sugar, chilli flakes and 125 ml water and bring to the boil, then reduce to a simmer.
Using damp hands, squeeze the filling out of the sausage casings, if necessary, and shape into walnut-sized balls. Add these to the simmering tomato sauce. Simmer the meatballs in the sauce for 5 minutes, shaking the pan often to move the meatballs around so that they cook evenly.
Bring a large saucepan of lightly salted water to the boil and cook the penne for 8–10 minutes, until tender.
Meanwhile, to make the garlic bread, preheat the oven to 170°C / Gas 3. Cut the baguette into 4 equal portions, then cut it in half through the centre. Combine the butter, garlic and parsley in a small bowl and add a little salt. Spread the mixture on the cut side of the baguette, wrap in foil and bake in the preheated oven for about 10 minutes.
Drain the pasta well and return it to the warm pan. Season the meatball sauce to taste with salt and pepper. Spoon the pasta onto serving plates and top with meatballs. Sprinkle with grated Parmesan cheese and serve with the warm garlic bread on the side.