Wash mushrooms first removing any dirt.
You can cut the mushrooms how you like. I personally slice the mushrooms thinly, cutting in one direction only. Cross chopping, I think takes away from the mushroom presentation.
Slice the onions lengthwise into strips. Not chopped
Add butter to a medium-sized pan 2 tablespoon or less. The medium sized pan will help with the sauce reduction(more surface area). Add the mushrooms and onions with a pinch of salt and cook them down. About 10 minutes. Try to caramelize the onions
Next, add the red wine and reduce down. Just make sure you cook off the alcohol.
Add the beef stock next with the rest of the fresh thyme and 2 teaspoon of mustard and allow to reduce. Once close to sauce consistency mount the sauce with butter. As much as you like. Min of 1 tablespoon