Clean the mushrooms and trim the ends off the stems. Slice the mushrooms thinly and sprinkle them with the lemon juice.
In a saucepan, melt the butter over low heat and add the garlic and mushrooms, reserving a few slices for garnish. Cook, stirring gently, for 5 minutes, then stir in the flour. Add the water and bouillon cubes and bring to a boil. Lower the heat to medium and cook for 15 minutes to reduce.
Season with a little salt and pepper, reduce the heat to low, and stir in the milk and grated Parmesan.
Transfer to a blender and puree until smooth. Blend in the crème fraîche.
In a hot skillet with a little olive oil, brown the reserved mushroom slices.
Pour the soup into a tureen and top with the mushroom slices. Sprinkle with the shaved Parmesan, parsley, and a few grinds of pepper and serve right away.