Adjust oven rack to highest position and turn on oven to broil. (The heating element should be 5-6 inches away from your pan).

Cut the onion into eights and pull apart the the onion layers. Slice off the root part of the garlic bulb to expose the raw cloves. Place the onion pieces and garlic on a baking sheet lined with foil, and broil for 10 minutes. Use tongs to flip garlic and broil for an additional 5 minutes. Set aside to cool.

While onion and garlic are broiling, strain tomatoes in a mesh colander. Use a rubber spatula to press out excess liquid. Set aside.

When onion and garlic have cooled to the touch, separate the garlic cloves from the bulb and then squeeze each clove from the non-cut end to release the roasted garlic.

Add the onion and peeled garlic cloves to a food processor fitted with blade attachment. Process for 5 seconds, scrape down the sides of the bowl, and then process for another 5 seconds. Next add the tomatoes from the strainer and 3 chipotle peppers. Process for 5 seconds to combine ingredients.