In a small bowl cover the dried mushrooms with 1 cup of boiling water. Allow to sit about 20 minutes. Keeping the water, strain out the mushrooms and chop.
Thinly slice the white or brown mushrooms. In a large saute pan, heat oil and add onions, cook over medium heat for 30 minutes, stirring constantly until caramelized and brown. Add in all of the mushrooms. Sauté until reduced and lightly browned. add herbs and pepper. Deglaze the pan with the hot vegetable stock. Add the porcini mushroom liquid now too. Scrape the bottom of the pan to get the nice brown bits up. Lots of flavor there! Bring to a gentle boil. Allow to simmer for 15 minutes.
In a small bowl or measuring cup, mix the flour with 1/2 cup COLD water. Stir until smooth. Making sure the gravy is at a low boil, stir in the flour mixture, stirring the graving until its completely combined. If you don't, you will get a lumpy gravy. Allow to cook an additional 5-10 minutes to thicken and cook the flour.
Serve hot. Store for 1 week in the fridge.