Roughly chop tomatoes and onions. Peel and smash the garlic cloves.Add ingredients to a blender and process on medium setting until completely smooth, for about 60 seconds. This should yield about 6 cups of raw sauce.
In a large, heavy-bottomed pot, heat 2 tablespoons of oil over medium heat until shimmering. Add the prepared tomato mixture and bring to a gentle boil. Cook for approximately 15 minutes, stirring frequently to prevent scorching and ensure even cooking.Reduce the heat to low and allow the sauce to simmer gently for an additional 30–45 minutes, stirring occasionally. Continue cooking until the sauce has reduced by about half and develops a deep orange hue. At this stage, the sauce should taste fully cooked, with no raw tomato flavor remaining. Total cooking time will be approximately 45–60 minutes.PRO TIP: Use a generously sized pot to allow for bubbling and splattering during cooking.
Allow the sauce to cool to room temperature, about 30–45 minutes, before transferring to clean, airtight containers. Store in the refrigerator for up to 4–5 days, or freeze for up to 6 months.
Fresh vs. canned ingredients: Using fresh tomatoes, onions, and garlic will result in a more flavorful sauce. However, you can also use canned diced or whole tomatoes to make this sauce when tomatoes aren't in season or available. Avoid using fire roasted, or Italian flavored canned tomatoes for this recipe.
Roast the tomatoes and chilies: Roasting the tomatoes and chilies before blending them can add depth and smokiness to the flavor of the sauce.