Preheat your oven to 320°F (160°C) and line a springform pan with parchment paper.
In a large bowl, combine the quark, eggs, whey protein, almond flour, sweetener, baking powder, vanilla extract, and lemon zest.
Mix everything until smooth and creamy.
Pour the batter into the prepared pan and smooth the top with a spatula.
Bake for 35–40 minutes, until set and lightly golden.
Let the cake cool completely before removing it from the pan.
Garnish with fresh berries if desired and serve chilled.