Heat the broiler to high.
Place the onion, garlic, jalapeño, and tomatoes on a sheet pan. Toss with the oil and 1 teaspoon kosher salt. Transfer to the oven and broil for 10 minute or until the vegetables are nicely charred.
Transfer vegetables to a food processor. Add the chipotle, the lime juice, and ½ teaspoon kosher salt. Add ½ cup water. Purée until smooth. Taste. Mixture should taste both sweet and spicy with a nice balance of acidity. Adjust with salt to taste. Add water by the ¼ cup till it is thinned to the right consistency — it should have some texture because of the vegetables, but it should be pourable as well.
Use in your favorite enchilada recipe or store in the fridge for up to 1 week or freeze for up to 3 months.
