Heat the oven to 450ºF and position the rack in the middle. Line a shallow baking sheet with parchment paper and place tomatoes, onions, jalapeño and garlic on the parchment. Transfer to the oven and roast until charred and soft, 15-20 minutes for onions and garlic, and 30 to 35 minutes for tomatoes.
Transfer the vegetables to a blender and add cilantro, chipotle and salt. Purée the vegetables on medium speed until uniform and, with the motor running, slowly add oil to emulsify until incorporated. If the consistency of the salsa is too thick, add water, 1 tablespoon at a time, to adjust consistency to a smooth purée. Taste and adjust seasonings and let cool before serving, or cover and refrigerate until needed.