In a stockpot over medium-low heat, saute the leeks in butter for 10 minutes. Gently stir in the potatoes and saute for 10 minutes longer, or until tender. Add the mushrooms and saute for 5 minutes longer. Gently stir in the flour and paprika, and simmer for 2 to 3 minutes. Add the chicken stock, increase the heat to a fast simmer, and cook for 20 minutes. Add the cream; stir and cook until slightly thickened. Puree in batches in a blender or food processor. Add salt and pepper to taste, stir in thyme, and serve.