Wash tomatoes and slice off the stem end. Scoop out pulp, chop fine and reserve. Salt the inside of each tomato and invert onto paper towels.
Combine sour cream, chopped tomato pulp and egg yolk.
Melt 2 Tbsp butter in sauté pan and sauté onion until soft, add mushrooms and sauté until they are tender. Add sour cream mixture to the onion-mushroom mixture, stir in breadcrumbs and seasonings and cook until slightly thickened, about 3-4 minutes. Season with salt and pepper.
Fill each tomato and bake uncovered at 375° for 25 minutes.
Sprinkle with Parmesan cheese in the last 5 minutes of bake time.
Serve warm.