Season steak generously with salt and pepper.
Heat butter in a large skillet (preferably cast iron) over medium-high heat.
Add cube steaks and cook on medium high until well browned on each side, about 4 minutes per side. Remove steaks from pan when browned and set aside.
Add second half butter to pan and melt.
Add onions and mushrooms and sauté on medium, stirring often, until onions are soft and mushrooms have browned slightly.
Add garlic and cook until soft and fragrant, another 2 minutes.
Stir in beef stock, stirring to deglaze pan as you add stock.
Bring to a boil and let reduce, about 4 minutes.
When gravy has thickened, add steaks back to pan.
Toss in gravy to coat.
Serve immediately.
Leftovers may be stored for up to four days in an airtight container in the refrigerator - up to three months in a freezer.
If freezing, let thaw naturally in the refrigerator overnight and reheat in pan on the stove. Add up to 2 cups beef stock to make more gravy if needed.