Finely chop reserved mushroom stems. In sauté pan, heat canola oil. Add chopped mushroom stems, onion, seasonings and garlic. Saute 5 minutes. Add prosciutto. Continue to sauté 5 minutes more. Turn heat to low and add cream cheese. Stir until mixture is well blended. Remove from heat and allow mixture to cool slightly.
Preheat oven to 350°F. Generously stuff each mushroom cap and place in baking dish. Bake 15 to 20 minutes. Finish under broiler until tops are golden.