Add onion to a pan and cook in oil for about 2 minutes until onions are translucent.
Remove half of the onions into a large bowl (to make room for your beef).
Mix the ground beef, onion salt, and pepper.
Make patties and roll them in flour.
Place your patties in a pan and brown both sides (you don't have to cook them through and through. They'll finished cooking in the gravy you'll make next).
Remove the patties, put them on a plate, and set aside.
Reduce the pan to low heat, and add butter.
Once your butter has melted, whisk in the 3 tbsp. flour, beef stock, water, spicy mustard, and Worcestershire sauce until smooth.
Increase the heat to medium.
Stir the ingredients, add in the onions you set aside earlier, then add the steaks into the gravy (along with any juices that pooled on the plate).
Cook for 5-10 minutes, or until your gravy thickens and steaks are cooked through. If your gravy thickens too fast, just add more water.
When your gravy is just about the right thickness, remove the steaks and put them back onto a plate.
Taste your gravy, salt and pepper to taste (when it's perfect, you'll know).
Now just serve up your Salisbury steaks, topped with your delicious gravy!