|Pre-heat oven to 350'F. Clean caribou heart under running water, removing any blood clots. Set aside.
In a bowl, mix together cooked rice, salt, pepper, thyme, oregano, chopped onion, dried vegetables, and canned mushrooms. Add enough water to moisten. Fold in softened butter.
Stuff mixture into heart pockets and place in a roasting pan. Bake covered at 350'F until brown, approximate 2 to 3 hours.