Heat the olive oil over a medium heat, once hot add the onions and garlic, cook, stirring occasionally until the onions have gone translucent,
Add the chopped tomatoes and cook until they have softened and started to break down, season with sea salt.
Add in the balsamic vinegar and sugar and cook for 3-4 minutes until the sugar has full dissolved.
Remove from the heat but keep warm, stir in the lemon zest and season if required
Roughly chop the basil and add it to the sauce just a few minutes before serving.