Preheat oven to 350 degrees. 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 to decorate the top of the cheesecake.

More about digestive biscuits

I find these at my local Kroger or even HEB grocery store.  They are closely related to graham crackers.  These are a semi-sweet biscuit that originated in the United Kingdom.  They were believed to have antacid properties because they contained sodium bicarbonate when they were first created.  These contain brown wheat flour, sugar, malt extract, vegetable oil and other ingredients.  They tend to be made with sugar and not high fructose corn syrup.  They are typically eaten with coffee or tea.

King Arthur Flour has a recipe for Digestive Biscuits

Swirl and Scramble has a recipe for Digestive Biscuits too

Why I liked using the digestive biscuits in this recipe is because I wanted a different texture than the normal graham cracker crust recipe.  So this gave me a finer grain crust, instead of the typically coarse grained graham cracker crust.  Also, I didn't need to add any sugar to this crust because the biscuits were already sweet.  You can use either the digestive biscuits or the graham crackers, both will make a suitable crust for this mandarin orange cheesecake.