Gather your mise en place; get together all of the equipment needed for the recipe and gather your ingredients.

In a blender or food processor, purée tomatoes, onion, garlic, apple cider vinegar, sugar, oregano, and salt.

Pour tomato mixture into a large pot, and heat to medium heat. Cook for about 20 minutes until mixture has thickened. Partially cover the pot with a lid to avoid it sputtering and burning you.

Let cool, and serve alongside pupusas. Salsa roja will keep well in a covered container in the fridge for up to two weeks.