Melt the butter in a skillet over medium heat. Stir in the mushrooms and onion, and cook until tender. Sprinkle with the flour. Reduce heat to low, and continue cooking 5 minutes. Mix in sour cream and lemon juice, and season with salt and pepper. Transfer to a small casserole dish, and top with Parmesan cheese.
Bake 15 minutes in the preheated oven, until bubbly.