Heat the olive oil in a large soup pot. Add the leeks and garlic and cook over moderately low heat, stirring, until lightly browned, about 5 minutes. Thinly slice the fresh mushrooms and add them and the reconstituted mushrooms to the pot. Cook, stirring, until the liquid has evaporated, about 10 minutes. Add the reserved mushroom liquid and the stock, oat groats, carrots, parsnip, balsamic vinegar, thyme and rosemary and bring to a simmer. Cover and cook over moderately low heat until the oat groats are tender, about 35 minutes. Season with salt and pepper and stir in the parsley. Serve piping hot.