In a small saucepan, simmer the mushrooms in 2 cups water until soft about 40 minutes. Remove mushrooms with a slotted spoon. Strain the cooking liquid and set aside. Pat the mushrooms dry with paper towels and finely chop. Melt the butter in a small skillet over medium heat. Add the mushrooms and onion and saute, stirring for 15 minutes. Sprinkle with flour, stir and cook for 1 minute. Stir in 1/4 cup of cooking liquid and simmer for 2 minutes. Stir in sour cream ans simmer for 5 minutes. Add garlic and season with paprika, salt and pepper. Simmer for 2 minutes. Preheat oven to 350 degrees. Heat oil in large skillet and lightly toast bread on both sides. Transfer to baking dish. Heap some of the mushroom mixture on each bread slice. Sprinkle with parmesan cheese and bake for 10 minutes. Sprinkle with parsley and serve.