Combine flour, garlic salt, and black pepper in a pie plate or shallow dish. Dredge both sides of cubed steak in flour mixture.

Heat oil in a large skillet (preferably cast iron) over medium-high heat. Add steak (cooking in 2 batches if necessary). Cook 5 minutes per side. Remove steak from skillet and set aside.

If you want to thicken the gravy some, add a few teaspoons of leftover flour from dredging the steak to the drippings in the pan.

Add onion, broth, and onion soup mix to the drippings in the pan. Use a wooden spoon to scrape up any browned bits stuck to the bottom of the pan.

Bring to a simmer and cook until onions are soft.

Return steak to pan to warm. Serve with mashed potatoes.