Liberally season all sides of the roast with salt and pepper and place in a large Dutch oven over high heat. Quickly sear all sides of roast to achieve nice brown crust.
Reduce heat to medium low and add enough water to to cover the bottom of the pan with about 1 inch of water.
Add quartered onion, smashed garlic and chopped jalapenos. Cover and simmer for 3-4 hours until roast is very tender. (check water level periodically and add as needed)
Once the roast is very tender, add beef bullion cubes, lightly mash up vegetables with a fork and simmer for another 30 minutes or so to reduce liquid to one half (i.e. reduce until there is about 1/2 ince of liquid in the bottom of pan)
Wash and shred lettuce. Divide equally among serving plates.
Arrange avacado, onion slices and tomoato slices on top of lettuce mixture.
Once the liquid has reduced, remove from heat and transfer roast to a large bowl. With two forks, shred the beef entirely.
Place equal portions of shredded beef on top of salad mixture and add crumbled feta cheese.
Mix 2 parts Italian dressing with 1 part cooking liquid and dress salads with the mixture.
Serve with warm crusty bread or just plain saltine crackers.