Soak the dried mushrooms in boiling water for 20 to 30 minutes to rehydrate them. Drain off the liquid but reserve it for the gravy. Slice the mushrooms into smaller pieces.
In a large pan over medium heat, melt the butter and gently cook the diced red onion until softened, about 5 minutes. Stir in all the mushrooms.
Sprinkle over the flour; stir and cook for a few minutes before pouring in the reserved mushroom liquid and wine. Bring to the boil; add stock and fresh herbs and continue to simmer so the liquid reduces and thickens a bit (about 5 or 10 minutes). It will still be quite thin.
Optional: Before you're ready to serve, you could strain the gravy if you prefer it to be completely smooth.