Preheat the oven to 350 degrees F (180 degrees C).
Clean the Cremini Mushroom (15) and remove the stems.
Finely chop 3 of the mushrooms and set the others aside. Finely dice the Onion (1/2), mince the Garlic (1 clove) and chop the Sage Leaves (3). Set aside.
When the oven is hot, toss the remaining whole mushrooms with Olive Oil (2 tablespoon) and roast for 15 minutes.
Combine the Parmesan Cheese (1/4 cup) and Ricotta Cheese (1/4 cup) in a bowl.
Brown the Lamb (0.5 pound) over medium heat, about 3 to 5 minutes. Remove with a slotted spoon leaving behind the rendered fat.
Saute the onion over medium heat in the fat for about 5-7 minutes, until translucent. Stir in the garlic and cook for a minute.
Stir in the diced mushrooms and cook for another 5 minutes. Then stir in the Breadcrumbs (to taste), Butter (to taste), and sage leaves. Turn off the heat and mix lamb back into the mixture.
Place roasted mushrooms bottom side up on a sheet pan. Turn on the broiler in your oven.
Stuff your mushrooms with your lamb mixture. Top the stuffed mushroom with a small spoonful of cheese mixture. Then place in the oven for 5 minutes or until cheese is slightly crispy.