Place pot roast in large pot.
Place sliced onions on top of roast.
Pour beef broth over meat and onions.
Bring to a boil, reduce heat and cover with a tight lid. Allow to simmer for 3 hours.
Place meat on large cutting board and use a fork to shred meat along the grain. Place in bowl and stir in ½ cup of meat broth. Stir, cover and keep warm.
Preheat oven to 300°.
Stir together mayonnaise and ancho chili pepper in small bowl.
In separate bowl, stir together all the dressing ingredients.
Split buns and toast slightly on oven rack for a couple of minutes. Watch closely so they don't burn.
Spread bottom of bun with refried beans.
Spread top bun with ancho chili mayonnaise.
Spoon about ⅔ cup hot meat onto bottom of bun.
Drizzle dressing over meat.
Top with shredded cheese and avocado slices.
Serve warm!