Step 1: Start by washing your fresh tomatoes and onions. Peel the onions and chop them into small chunks. Mince the garlic cloves, and deseed the dried chile de árbol. Cut each of these ingredients into smaller, more manageable pieces that will be easier to blend.
Step 2: In a large saucepan, heat a generous amount of vegetable oil over medium heat, adding the diced onion and minced garlic once the oil is warm. Sauté these ingredients for about 5 minutes, until they are fragrant and lightly browned, then add the chopped tomatoes and stir everything together.
Step 3: Reduce the heat slightly and allow these ingredients to cook for another 5 minutes until they have softened.
Step 4: Once the mix has reached your desired consistency, stir in your deseeded chile de árbol pieces along with salt, vinegar, and white pepper powder to balance out the flavor. Cook the mixture for 3 more minutes and then remove it from the heat, and allow it to cool down.
Step 5: Once the mixture is cooled, add it to a blender and blend it until you get a fine smooth paste. Taste the sauce and adjust for seasoning.