Preheat oven to 170°C and line a springform pan with parchment paper.
In a large bowl, mix the quark, cream cheese, erythritol, vanilla, eggs, and lemon zest until smooth.
Stir in the cornstarch (or almond flour) until combined.
Pour half of the mixture into the prepared pan, scatter half the mandarins, then cover with the remaining mixture.
Top with the remaining mandarins and gently press them in.
Bake for about 50 minutes until the edges are set but the center is still slightly wobbly.
Turn off the oven, open the door slightly, and let the cheesecake cool inside to avoid cracks.
Chill in the refrigerator for at least 3 hours before serving.