Preheat your oven to 350º. Grab a cake pan and fill it in 1 inch high with water. Place this in the oven while it preheats.
Take your cold stick of butter and rub the inside of the springform pan so it is buttered up. Melt that same stick of butter in a small pan on low heat. Take it off the heat as soon as almost all the butter is melted (it's ok if there are little solid bits).
Grind your tea biscuits in the food processor or put them in a giant ziplock bag, and bang on it with a rolling pin til it's finely ground.
Transfer the biscuit crumblies into a medium bowl and add the brown sugar and melted butter. Mix with a fork until butter is dispersed and crumbly biscuit BALLS form.
Transfer the buttered crumblies to your springform pan. Press the mixture against the floor of the pan and a centimeter or so up the sides. The more compact the better! Your crust is ready for bake-off.
Dump your softened cream cheese, room temp sour cream, and sugar into a large bowl. Beat this on high til there are no sugar granules left (yea, stick a finger in it to check).
Crack three eggs in a small bowl. Add vanilla extract to small egg-filled bowl.
You're going to add one egg into the cream cheese batter at a time. Mix. Add another egg. Mix. Repeat. Don't dump all the eggs in at once! The batter should be smooth before each egg addition.
Grind your pistachios in the food processor and finally, add the ground pistachios and ground fennel into the batter.
Pour batter into the "crusted" springform pan.
Is your oven preheated and steaming from the water bath inside? Great — put your cheesecake in! Bake for about 35-45 minutes (until just golden brown on top). Wobbly centers are welcome here.
For the topping: Whisk heavy cream in your mixer until soft peaks form. Add vanilla extract and powdered sugar and continue to whisk for one minute (stiff peaks). With a rubber spatula, plop whipped cream onto your fully-cooled cheesecake. Slice blueberries in half to garnish.