Cut meat from bones into chunks about 3/4". Heat 1/4 stick of butter in a large steel or aluminum pot. Add 1 tsp olive oil to retard burning. Brown deer chunks until well-seared, then add sherry or port and cook for 10 minutes more. Remove from pot and set aside. To the liquid remaining in the pan add pearl or chopped onion and brown until golden. Add tomato paste, stirring in well. Add flour and stir into mixture. Return deer to pot, adding stock. Add 1/3 the red wine, bring to a boil, then simmer for 3 hours or until tender, adding wine by 1/3 until used up. Add mushrooms 30 minutes before serving. Serve over buttered noodles or rice.