Set your pressure cooker to Saute and add the oil. You can also just use a pan as well. Once the oil is hot, add onion and cook until tender, about 5 minutes.
Add ground beef and chop with a spatula to break up the meat into small crumbles. Season with salt and pepper.
Once half the meat has browned, add garlic, Worcestershire and mustard. Stir until combined, and cook the meat is no longer pink.
Take a piece of bread and toast it. Pile meat onto the roll or bread, top with pickles and cover. Repeat until you have four sandwiches.