Steps

1. In a small saucepan, bring the water to a boil. Remove it from the heat, add the dried mushrooms, and let stand at least 1 hour, or until the mushrooms are soft and you feel no hard areas when you press them between your fingers.
2. Set a fine strainer over a bowl. Strain the mushrooms, reserving the soaking liquid. Rinse the mushrooms under cold running water to remove any grit or sand, and drain again. Dry on paper towels. Chop the mushrooms (you want about ¼-inch pieces) and set aside.
3. Line the strainer with paper towels and set it over a bowl. Strain the mushroom broth into the bowl to remove any grit. You should have about 1 cup broth; set it aside.
4. Heat the oil and butter in a saucepan over low heat. Add the chopped onion and cook until it is translucent and soft, but not brown, 8 to 10 minutes.
5. Add the tomato purée, the chopped mushrooms, and the strained mushroom broth. Season with ½ teaspoon salt and ¼ teaspoon pepper. Cook, uncovered, at a gentle simmer, stirring occasionally, until the sauce is slightly thickened, about 45 minutes. Taste for seasoning and add more salt and pepper if the sauce needs it.