Preparation
Heat a frying pan over a medium heat. Tip in the mince and cook for about 5 minutes until well browned, breaking up any lumps with a wooden spoon. If you think that there is any excess fat in the liquid that comes out of the meat, then drain it off but generally if you use a lean steak mince this will not happen.
Using a small sharp knife, cut the onion in half and peel off the skin, then put each half on a chopping board and cut into slices, keeping the root intact. Then make 2 horizontal cuts one above the other and finally, chop down across the width of the onion. Cut the red peppers in half and remove the seeds and cores. Cut into dice and add to the mince with the onion and cook for another few minutes until softened, stirring.
Pour the sauce into the mince and vegetable mixture, stirring to combine. Season lightly with salt, if using and plenty of freshly ground black pepper. Cook for 30 minutes until the peppers are really soft and the sauce has reduced and thickened, stirring occasionally with a wooden spoon to ensure it does not stick to the bottom of the pan.
Meanwhile, cook the spaghetti in a pan of boiling water with a pinch of salt for 10-12 minutes or according to packet instructions. Drain in a colander in the sink. Divide the spaghetti among wide rimmed bowls and spoon the Bolognese sauce on top to serve.
Prepare Ahead
The Bolognese sauce can be made in advance and kept in an airtight container for up to 3 days in the fridge. It also freezes very well for up to one month. Simply defrost thoroughly and either heat once in the microwave or over a gentle heat in a saucepan. Serve with freshly cooked spaghetti.