In a large saucepot with water, mix beef shanks, onions, and peppercorns.
Wait for the mix to boil then, reduce heat to low, and cover the pot.
Cook beef until fork tender. Transfer the beef in another saucepot.
On the used pan, melt butter and sauté garlic, onions, and sliced mushrooms.
Add in flour and mix well. After, add liquid seasoning and beef stock.
Stir until well combined. Wait for the mixture to simmer.