In a blender, add the tomatoes, chipotle, onion, garlic, and onion.
Blend until smooth.
In a large pan, heat up the oil.
Stand back and slowly add some of the tomato mixture.
Immediately add a little more of the tomato mixture.
Then add the remainder of the mixture.
Bring to a simmer and reduce heat.
Cook on low heat for 15 minutes.
The sauce will have thickened up.