Lightly coat a casserole dish (which will fit inside the slow cooker) with vegetable oil.
Tear off a large sheet of kitchen foil and cut in half, lengthwise.
Fold each piece lengthwise into thirds.
Crisscross the foil strips and place the dish in the centre of the crisscross and set aside.
For the filling, in a large bowl beat the cream cheese, sugar, orange juice, flour, and vanilla until combined.
Beat in the sour cream until smooth.
Beat in the eggs just until combined.
Stir in the orange zest.
Pour the filling into prepared dish.
Cover the dish tightly with foil.
Pour warm water into the bottom of the slow cooker.
Using the ends of the foil strips, transfer the dish to cooker.
Leave foil strips under the dish- this creating a way to lift it out later.
Cover and cook on high for 2 ½ hours.
Using the foil strips, carefully remove the dish from the cooker and discard foil strips.
Cool completely, uncovered, on a wire rack.
Cover and chill for 4 to 24 hours before serving.
Garnish with orange slices and crystallized ginger if using.