Crush the digestive biscuit cookies in a food processor, process cookies until they are crushed into fine crumbs. Pour cookies into a bowl, and add 6 tablespoons of melted butter and nutmeg and stir to combine. Press the crust into an 8-inch spring-form pan. Bake crust in for approximately 12 - 15 minutes. The crust should be a golden brown color.
In a medium-sized bowl combine cream cheese and sweetened condensed milk. Add 1 teaspoon lemon juice, and one teaspoon vanilla stirring into the cream cheese. Mix cheesecake mixture until very smooth, this can be done by hand or with a mixer. Peel two of the mandarin oranges, and cut the segments into small pieces over the bowl with the cheesecake mixture.
You will want any of the juices to fall into the cream cheese batter. Stir the orange pieces into the cream cheese. Spread the cheesecake batter over the crust, and refrigerate for at least 8 hours before serving. Just before serving, peel the last orange and use segments to decorate the top of the cheesecake.