Using medi;um heat, add 2 tbsp. butter to the pan; once the the butter has melted. add the sliced mushrooms and onions.
Cook the mushrooms for 2 minutes until golden brown;
Stir the mushrooms, coating each one in the flour; continue cooking for another minute.
Remove the mushrooms from the pan; set aside.
Using the same skillet, over medium heat, melt the remaining 4 tbsp. butte and add the sliced onions.
Cover partially and let them simmer, soften, and reduce for about 25 minutes; stir every few minutes and reduce heat if they start cooking too quickly.
Once onions are cooked, add the mushrooms; then sprinkle the flour over the mushrooms and onions; stir to complete combine.
Add the beef stock, stirring constantly to continue to incorporate the flour.
Add the steak patties to the sauce mixture; simmer, uncovered, for an additional 5 minutes or until fully cooked and sauce has thickened.