Heat a tbsp of olive oil in a large saucepan. Add in the sliced onions and saute until they turn translucent.
Add in the rest of the ingredients and turn the heat to medium-high, place the lid on and let your stew simmer for 45 minutes.
Take the lid off and let it reduce down for a further 15 minutes. You can let it reduce longer if you want a thicker stew or combine a couple of tablespoons of water with 2 tablespoons of cornflour and transfer it the stew, stirring to combine (this will thicken your stew more quickly).