Place backstrap in a container big enough to cover it in milk. Let the venison soak in the milk overnight. Place all the marinade ingredients into a large bowl, whisk until well incorporated. Remove the venison from the milk , place in a large bowl, pour the marinade over the venison, let marinade at least 8 hours. After 8 hours of marinating, remove the venison from the marinade, reserving marinade. Cut into slices or small pieces. Grill 6 to 8 minutes on each side.Place reserved marinade in a medium saucepan. Cook until it comes to a boil, reduce heat to a simmer. Combine 1 Tablespoon cornstarch and one Tablespoon water in a small bowl, stir the mixture into the marinade. Cook until thickened slightly. Serve with white rice and top with sauce.