Preheat the oven to 375 degrees F (190 degrees C).
In a medium bowl, toss together the cheese and mushrooms. In a separate bowl. mix together the ground beef and onion soup mix. Divide the beef into 12 to 16 balls, then form them into small, thin heart shaped patties. Press an indention into the center of half of the hearts, and fill with the cheese and mushrooms. Place another patty on top, and press to seal the hearts around the filling. Place in a shallow baking dish.
Bake for 30 to 40 minutes in the preheated oven, until beef is well done, and filling is hot.