In a saucepan, bring water to a boil. Dribble in the cornmeal in a steady stream, whisking constantly. Add salt. When polenta begins to boil, reduce heat the low. Cook, stirring frequently, until it is thick and pulls away from the sides (30-40 minutes). Oil one 8 inch baking pans. Pour polenta into the prepared pan. Cool until firm; cover and refrigerate.
In a large skillet over medium heat, melt the butter with olive oil. Add the garlic and cook, just until lightly golden. Stir in the mushrooms and rosemary or thyme, salt and pepper. Cook, stirring frequently, until the mushrooms are browned, about 10 minutes.
Add the chopped tomatoes and cook until the juices have evaporated. Stir in the stock and cream and reduce slightly. Keep warm.
Cut polenta into 8 pieces and brush both sides with olive oil. Arrange on a heavy baking sheet. Broil, turning once, until crisp and golden on both sides. Place polenta on plate and top with the mushroom mixture, dividing equally. Serve immediately. Makes 4 servings.
From Williams-Sonoma, Savoring Italy, Michele Scicolone, Time-Life, 1999.