You'll start by slicing the white onions thinly using a mandoline or a knife, then put them in a bowl with the salt and sugar and combine. This process will help the onions release their juice which will be useful when cooking.

Let the onions sit in the bowl for at least two hours.

Once the onions have released all their juice, melt the lard in a big pot, then add the onions together with their own liquid and cook for about 30 minutes.

Mix them every so often so that the onions can get golden and soft and won't stick to the pot. You can add a bit of water if the onions are starting to get stuck to the pot, just enough to get them to not do so.

After 30 minutes you can add the peeled tomatoes and let everything cook for a further 1 hour on low with the lid on, mixing every so often just like before. Just like before, you can also add some water to keep the sauce runny and not disappear fully.