Mince mushrooms, then mix with butter, minced onion, salt, and pepper in a small bowl. Reserve half of butter mixture, and divide other half over the 4 filets.
Heat a large skillet over medium-high heat. Place steaks, butter side down, in hot skillet. Spread remaining butter mixture over steaks. Cook to desired doneness, turning once. Transfer filets to a plate, cover loosely with foil, and keep warm.
Return skillet to medium-high heat. Stir in reserved mushroom liquid, and scrape up any browned bits from the bottom of pan. Add jelly and lemon juice, and simmer until liquid is reduced by half. Stir in ice cream, and cook until sauce is thickened, about 2 minutes. Pour over filets, and serve immediately.