Place the beef chuck roast pieces in the crock pot.
Add the sliced onion and minced garlic on top of the beef.
Pour the beef broth over the ingredients in the crock pot.
Sprinkle the ground cumin, chili powder, smoked paprika, salt, and black pepper evenly over the beef and broth.
Cover and cook on low for 6 hours, or until the beef is tender and easily shredded with a fork.
Once cooked, remove the beef from the crock pot and shred it using two forks.
Return the shredded beef to the crock pot and stir to combine with the juices.
Heat 1 tablespoon of vegetable oil in a large skillet over medium heat.
Place one flour tortilla in the skillet, sprinkle 1/4 cup of cheddar cheese and 1/4 cup of Monterey Jack cheese evenly over half of the tortilla.
Spoon about 1/3 cup of shredded beef over the cheese, then sprinkle another 1/4 cup of cheddar and 1/4 cup of Monterey Jack cheese on top of the beef.
Fold the tortilla in half over the filling and cook for 2-3 minutes on each side, or until the tortilla is golden brown and the cheese is melted.
Remove the quesadilla from the skillet and keep warm. Repeat the process with the remaining tortillas, cheese, and beef, adding more oil to the skillet as needed.
Cut each quesadilla into wedges and serve warm with sour cream, salsa, and chopped fresh cilantro if desired.