Soak the porcini mushrooms in warm water to cover for at least 45 minutes. Carefully remove from liquid & rinse well under cold water. Chop roughly & pat dry.
Heat olive oil in a heavy saute pot & saute onion until it is translucent & soft. Add the garlic & all the mushrooms. Reduce heat to low & cook, stirring intermittently, for 20 to 25 minutes, until tender. Add the tomatoes, parsley, salt & pepper & cook for another 5 minutes.
POLENTA: Bring the salted water to vigorous boil in a large pot. Reduce heat to low & slowly sprinkle in the cornmeal in a thin stream, first whisking it in & then stirring constantly being careful to eliminate any lumps. Keep the water at a steady simmer & stir frequently. When it comes away from the side of the pot, after 20 or 25 minutes, then it is cooked. Check for salt. Allow to cool by pouring onto a baking sheet & patting until it is as smooth as possible.
Cut the polnta into slices that are 2" wide & 3" to 4" long. Brush lightly with the olive oil. Broil until they are firm & lightly crisp on both sides. Place a spoonful of the hot mushroom sauce on the top of each crostini & serve.
Can be served as an appetizer, as an antipasto, as a side dish or as a main dish.