Mix the beef, breadcrumbs, egg, onion, 1 tablespoon Worcestershire, salt and pepper gently and shape 8 oval patties.
Brown the patties in the oil in 2 batches for 3 minutes per side and transfer them to a 6-quart crockpot.
Brown the mushrooms in the same skillet for 6 minutes and scatter them over the patties.
Whisk the broth, flour and remaining Worcestershire until smooth, pour it into the crock, and cook covered on low for 5 hours until the patties reach 160°F.
Lift the patties onto plates and stir the mushroom gravy before spooning it over them.