Peel potatoes and place in baking dish, whole. Bake in 350° oven for 30 minutes. Cut off cap at top of potato. Scoop out center and fill with mushroom mixture made as follows: Fry onion and mushrooms in butter for 10 minutes. Add bread crumbs, egg yolks and seasoning. Fill potatoes, replace cap and return to buttered baking dish, cap side up. Bake for half hour at 350°. You may vary the stuffing with ground ham, ground cooked meat, riced potatoes.