Finely dice the onion and garlic. Heat some oil in a large pan and fry the onions and garlic until translucent.
Dice the potatoes and tomato. Add to the pan with the spices. Mix everything well and fry for another 1 minute so that the flavors can develop.
Add the remaining ingredients and let everything simmer for about 30 minutes over medium heat with the lid closed. To thicken, stir the cornstarch in a little cold water and stir in. Bring the curry to the boil again until it thickens.