In a medium sauce pan, heat 2 tbsp of vegetable oil, add diced yellow onion and saute until onion turn brown.
Add ketchup, followed by diced tomato and 1 cup of preserved juice mixture (from above).
Add salt, sugar, and ground black pepper. Bring the mixture to boil then reduce heat to simmer for 10 minutes.
Adjust tomatoes sauce with salt and pepper if needed.