Start by following the instructions on the packet of the spaghetti and cook for 8 minutes in boiling water.
Whilst the spaghetti is cooking, warm some olive oil in a pan and add in the onions, cook for 3-5 minutes and then add in the garlic, balsamic and cook for a further 3mins until softened and slightly browned.
Once everything is cooked through add in the sauce and peas and mix well. Add in a few spoonfuls of the pasta water, alongside the spaghetti.