Melt 1 tablespoon of the butter in a large skillet over medium heat. Add the onions and dried shiitakes. Cook until onions have softened, about 5 minutes, stirring occasionally.
Add half of the fresh sliced mushrooms and sprinkle with a pinch of salt. Cook until the mushrooms begin to turn golden brown.
Add the wine to the skillet and bring to a boil. Cook 3-4 minutes until liquid has reduced.
Add the thyme sprigs and 2 3/4 cups of stock, reserving the remaining 1/4 cup in a small bowl. Bring stock and mushrooms to a steady simmer and cook for 15 minutes or until liquid has reduced slightly.
Use a slotted spoon to remove the thyme and the fresh and dried mushrooms from the liquid.
Add the flour to the reserved stock and whisk until smooth. Pour the flour mixture into the stock mixture, stirring constantly until smooth. Cook on a low simmer for 2-3 minutes or until the gravy has thickened slightly. Remove from the heat.
Melt 1 tablespoon of butter in another skillet over medium-high heat. Add the remaining fresh mushrooms, season with a pinch of salt and cook until mushrooms are golden brown. Add them to the gravy.

Season the gravy with salt, pepper and the apple cider vinegar. When seasonings are adjusted, add the last tablespoon of butter and stir until melted.