Combine chuck roast, broth, tomatoes, onion, bay leaves, serrano chiles, lime juice, vinegar, oregano, salt, and pepper in a large pot pot over medium-high heat. Bring to a boil, reduce heat to low, and simmer until meat is tender, about 1 1/2 hours.
Remove roast from pot and shred meat with 2 forks.
To assemble tostadas, spread a layer of refried beans on tortillas, layer shredded beef, lettuce, tomatoes, cheese, sour cream and salsa.
