• Preheat your oven to 350℉.
• Prepare your baking dish: Grease and flour or cover the bottom with parchment and grease the sides.

Make The Shortbread Crust:

• Beat the butter and sugar in a medium bowl. Add the eggs, blend in, then add the rest of the crust ingredients and knead to form a soft dough.

  If you're non-electric, you can add the flour, baking powder, sugar and salt to the bowl, mix, cut in the butter, add the egg, and knead into a soft dough.
• Roll out the dough, or just press it into the form till you have the bottom covered evenly.

  If using a round form, make a thick bottom, or.divide the dough in half, cover the bottom with one half and make a long roll with the other half and pull the crust up the sides.
• Set aside and make the filling.

Make The Filling:

• If your quark is lumpy or you're using a substitute such as cottage cheese, whisk, process, or beat your cheese till it's smooth before making the batter.
• Separate the eggs and add the egg yolks into the large bowl. Put the whites into a one quart or bigger mixing container.
• Place the sugar, butter, flour, baking powder, vanilla, lemon juice and zest into the large bowl. Beat till smooth. Add the blended cheese and half the cream, and beat till creamy and well incorporated.
• Add a pinch of salt to the egg whites, and, using egg beaters or a hand mixer with clean beaters, beat the egg whites till very stiff and you can make a clean cut with a knife.
• Fold the egg white very gently into the batter to keep it nice and fluffy. I like to use a silicone spatula for this.
• Pour the batter into the prepared crust, smoothe the top and place it into the oven. Bake for about 45-60 min.
• Cheesecake is prone to cracking, but if you cut around the edge of the cake, between the batter and the pan about halfway through baking, it should help minimize cracking. I didn't do that, it's just cosmetic, but that makes it look nicer and more even too.

  If you're making a round cake with a crust around the sides, cut between the filling and the crust.

  Place back in the oven and continue baking.
• Your cake is done when the center is set up and a toothpick comes out fairly clean. Leave the cake in the oven for another hour with the door cracked slightly open.
• Take it out of the oven and cool completely.
• Do not refrigerate until it is completely cooled.
• Store in the refrigerator until use. It will be good for up to 5 days in the refrigerator. Freeze any leftovers for up to 3 months.

Make The Blueberry Sauce

• In a saucepan combine the sugar and starch and whisk it to break up any starch lumps. Add half of the blueberries, and the lemon juice and zest. Slowly heat it while stirring and once the blueberries release their juices, turn the heat to med-high to get it to simmer.
• With an immersion blender or a potato masher, mash the blueberries well. Be very careful when blending this very hot sauce to avoid splashing!
• Bring back to a simmer and simmer while stirring for 2 minutes to thicken it, then take it off the heat.
• If you don't like the bits in it, you can strain it through a sieve for a smoother sauce. Cool this down to room temperature and add the rest of the blueberries. Stir to mix well.
• Set aside till the cheesecake is cooled to room temperature and serve the blueberry sauce along with the cheesecake or spread over the top of the cake if you're serving a crowd. Keep refrigerated until use.
• You can make this ahead too and just place the topping and the cheesecake separately into the fridge until use.