Melt butter in a large skillet over low heat.
Add mushrooms and onion; cook, shaking the skillet often to toss until soft, about 5 minutes.
Add ground sirloin, salt, and pepper; cook over medium heat, breaking up and stirring ground sirloin until browned.
Off heat, add sour cream and sherry; cook over low heat, stirring often, until sauce begins to simmer.
Serve over hot cooked rice, buttered noodles (tossed with poppy seeds), or toast points; garnish with herbs.
