In a large skillet, heat the olive oil over medium high heat. Add the sliced mushrooms and cook until they release their liquid and begin to brown, stirring occasionally, about 5 minutes.
Add the butter, garlic, shallots & sage to the mushrooms and sauté until softened, 2-3 minutes. Stir in the salt and pepper. Sprinkle the flour over the mushrooms and stir until all the flour is absorbed in the butter and forms a paste.
Stir in the vermouth and simmer for one minute. Add the stock slowly, stirring constantly. Heat the gravy to a boil and continue to stir for one minute until the gravy thickens.. Add the worcestershire sauce and the gravy master, stirring until well combined. Remove from heat and pour into a gravy boat or bowl. Adjust seasoning to taste. Serve.
Notes
Leftovers can be kept for up to 5 days in an airtight container in the fridge.