Combine the bread crumbs, ground mustard, paprika, and 1 minced clove of garlic in a large bowl
Add the egg and hamburger to the bowl and mix by hand until thoroughly combined
Make four oval patties, about one-inch thick, with the meat mixture
Heat the vegetable oil in a large skillet over medium-high heat and then brown the hamburger patties, cooking them about 3 to 4 minutes per side
Transfer the patties to a plate, they will finish cooking in the gravy
Turn the heat down to medium, add the thyme and mushrooms, and cook until the mushrooms are browned, about 8 minutes
Add the butter, onions, and remaining minced clove of garlic and cook until the onions are translucent, about 10 minutes
Whisk the flour into a 1/4 cup of the beef stock to make a slurry
Add the beef stock, flour slurry, and Worcestershire sauce to the skillet and turn the heat up to medium-high in order to bring it up to a slow boil to allow the sauce to thicken
Nestle the patties into the mushroom and onion sauce and cook until the hamburger is fully cooked, about 10 minutes
Plate the individual patties and spoon the mushroom and onion gravy over the top, serve with mashed potatoes