Heat a large cast-iron skillet over medium-high and pour in the olive oil.
Once hot, add the chuck roast and brown each side for about 3 to 5 minutes just until it develops a nice crust.
Transfer the browned beef to a slow cooker, then sprinkle with beef bouillon powder and pour the red enchilada sauce over it.
Cover and let it cook on low for around 10 hours until the meat is tender and shreds easily.
After cooking, lift the roast out and pull it apart with two forks, then stir the shredded meat back into the cooking juices.
Set a skillet on medium-high heat to prepare for grilling the sandwiches.
Spread butter on one side of each slice of Texas toast evenly.
Place the bread butter-side down on the skillet, and while it cooks, build your sandwich by layering ½ cup shredded beef, about 1 tablespoon of diced onion, ¼ cup queso fresco, ¼ cup shredded mozzarella, and a sprinkle of cilantro on one slice.
Top with another buttered slice of bread, butter side up.
Lower heat to medium-low and cook for 5 to 7 minutes until the bread turns golden and the cheese begins to melt.
Flip the sandwich carefully and cook another 5 to 7 minutes until the second side is golden and the cheese is fully melted.
Remove the sandwich from the skillet using a spatula and repeat with the remaining ingredients.
Serve your birria grilled cheese sandwiches warm alongside some of the leftover cooking juices for delicious dipping.
