In a large skillet, heat olive oil over medium-high heat. Sear the chuck roast on all sides until browned, about 3-5 minutes per side.

Transfer the seared beef to a slow cooker. Add enchilada sauce and top with bouillon powder. Cover and cook on low for 10 hours until tender.

Once cooked, shred the beef using two forks and mix it with the juices in the slow cooker.

Preheat a skillet or griddle over medium-high heat for the sandwiches.

Butter one side of each bread slice. Place slices buttered-side down on the skillet.

Top the bread with shredded beef, onions, queso fresco, mozzarella, cilantro, and place another bread slice on top, buttered side up.

Cook for about 5-7 minutes on each side until golden brown and cheese is melted, flipping carefully.

Serve hot with the juices from the slow cooker as dipping sauce.