Place dried peppers in a bowl and fill with boiling water to cover. Soak 30 minutes. Meanwhile, cover bottom of the slow cooker with chopped onions, set aside.
In a large saucepan over medium-high heat, add oil. Season brisket generously on all sides with salt and pepper, and when oil is hot, place meat in the pan; quickly sear it on all sides, about 1 ½ minutes per side. Transfer brisket to the slow cooker on top of the onions; set aside.
In the jar of a blender, add peppers, garlic, chipotle purée, lime juice, chipotle chili pepper, ground cumin, oregano and brown sugar. Process for 30 seconds before slowly pouring in ½ cup beef broth; blend until smooth, scraping the sides of the jar if needed.
Pour sauce over brisket and spread to cover it. Pour the remaining beef broth around, close the lid and set the timer for 6 hours on High or 9 hours on Low. When time is up, serve immediately or proceed with your recipe.