Preheat oven to 325°.
Brown roast on both sides.
Sauté onions.
Add to the onions, cumin and chipotle. Cook until it is fragrant and the onions are becoming soft.
Add the roast back to the pot.
Add the sliced peppers and broth.
Place lid on pot and cook in oven for 3 1/2 hours.
Prior to roast finishing, prepare your toppings.
When roast is done, shred meat (in pot), make sure to wipe the meat on the sides of the pot to remove all the flavorful fond (tongs work nicely for this)
Prepare your tacos with the toppings I've suggested or your favorite ones.