For the ultimate experience, let the gravy sit in the pot, covered with occasional stirring, for about ten minutes before serving. The gravy thickens a bit during this time as the flavors of the mushrooms blend with the sauce.
Melt two tablespoons butter in a pan, add the mushroom slices, sprinkle in a teaspoon of salt and quarter teaspoon of pepper, and cook, stirring often, just until lightly browned.
Scoop out the mushrooms and pour whatever liquid remains in the pan over them. Cover and keep warm.
Melt the remaining four tablespoons butter in the same pan, add the flour all at once and stir until a thick paste forms (it will thin out after a minute or so).
Set the pan over medium heat and cook the roux with occasional stirring until it turns brown and emits a nutty aroma.
Stir in a quarter cup of bone broth until fully incorporated.
Add the remaining broth, a quarter cup at a time, stirring after each addition.
When the gravy is boiling gently, add the mushrooms (with the liquid), drizzle in the Worcestershire sauce, stir and leave to boil gently for two to three minutes.