Soak the ancho chilies in warm water for 15 minutes or until they are soft and pliable. Remove and discard the stem and seeds. Place the ancho chilies in a food processor. Add the crushed tomatoes, onion and garlic cloves. Pulse until everything is smooth and pureed. Stir in the brown sugar, salt and pepper.

Heat oil in a skillet on high heat. Pat the chuck steak dry with a paper towel to remove moisture and lay in the skillet. Sear the steak on each side, cooking about 1-2 minutes per side.

Place the chuck steak in a small baking dish. Pour the tomato and ancho chili puree over the steak. Cover the baking dish and cook in a 325F oven for 2 1/2 hours.

Remove the steak from the baking dish, place on a plate, and let cool slightly.

Shred the beef with two forks and place the beef in a bowl. Take 1/2 cup (or more, if desired) of the cooked tomato sauce from the baking dish and stir in with the beef.

Make the guacamole by removing the flesh of the avocados and put it in a bowl. Mash the avocado with a potato masher or fork until fairly creamy. It is ok if it is a little bit clumpy.Squeeze the lime juice and stir it into the mashed avocado.

Dice the tomato and stir it into the avocado.

Assemble the sliders by adding some shredded beef, guacamole and fresh cilantro to each bun.