Set a large skillet over medium heat and add the oil. Once hot, add the onion and tomatoes and cook, stirring frequently, for 5 minutes. Add the tomatillo salsa and garlic, cook 1 more minute. Stir in the chipotle and adobo and remove from the heat. Allow to cool slightly, then transfer to a blender along with the broth or water and carefully puree until smooth (alternatively, you can use an immersion blender to puree).