How to Make It
In a large nonreactive saucepan, heat the vegetable oil over moderately high heat. Add the white mushrooms, Portobello stems, onion and garlic and cook, stirring, until the mushrooms release their liquid, about 5 minutes.
Add the wine, soy sauce, dried mushrooms, salt, herbes de Provence and 6 cups of water and bring to a boil. Cover, reduce the heat to moderate and simmer until the liquid is reduced to about 4 cups, about 1 hour.
Pour the broth through a fine strainer into a heatproof bowl. Strain again, leaving any particles at the bottom of the bowl.