In a blender, combine 3 of the chipotles from the can, along with 2 tablespoons of the adobo sauce from the can, crushed tomatoes, garlic, and spices. Blend until smooth, and set aside.
Heat a heavy skillet on the stovetop over medium high heat, add a drizzle of cooking oil and sear the chuck roast, undisturbed until deeply brown; about 3 minutes per side. Transfer the roast to your slow cooker and surround with the sliced onions.
Pour the beef broth to the hot skillet and scrape all of the brown bits from the bottom of the pan. Pour the deglazed pan liquid over the meat and onions, then pour in the blender sauce and mix gently stir together.
Cook on low heat for 6-8 hours. Once the beef is fork tender, transfer to a cutting baord and shred, removing large pieces of fat. Transfer back into the slow cooker and add the juice of one lime (or more if you like) and the chopped cilantro and toss to combine. Turn the crockpot to the warm setting until ready to use.
How to Use: Stuff shredded meat into soft taco shells or crispy fried shells (instructions below), or use in burritos, quesadillas, taco bowls and more.
Fried Tortilla Shells
To fry the taco shells: Fold the flour tortillas in half and use 3 toothpicks to hold them in a taco shell shape. Heat a heavy bottom pan over high heat with enough vegetable oil to reach 2” deep. Lay the tortillas in the oil one side at a time and fry until crisp and slightly puffy, flip and cook the other side. Remove to a paper towel lined sheet pan.
Notes
Prep the night before: Brown and then marinate the meat in the sauce in the slow cooker insert and store overnight in the refrigerator. Place the slow cooker insert into the base and set to low in the morning. This will give the meat even more flavor and tenderness.
You can use a fat separator or a spoon to skim the fat off the top of the cooking liquid before mixing the beef back in, otherwise the finished meat will be greasy.
For alternative cooking methods, see the notes in this post.