Preheat the oven to the lowest temperature (between 150 F and 170 F).
Place a rack in the center of the oven.
Pour the buttermilk into a large baking dish such as a casserole dish - it is best if the dish has a lid (does not need to be tight fitting).
If the dish does not have a lid, loosely cover the dish with a sheet of foil.
Place the dish with buttermilk in the preheated oven and bake for 1 hour.
If not using a glass dish, use a spoon or other utensil to see if the curds are thick and set, they should hold together slightly.
The texture should be somewhat like a very soft tofu - not as set as tofu.
Once the curds have thickened, turn off the oven and leave the dish inside for another 30 minutes.
After 30 minutes, remove the dish from the oven and leave it on the counter to cool to room temperature.
Line a fine mesh sieve with a layer of cheese cloth and place over a large bowl.
Scoop the the curds into the lined sieve. Then carefully pour the remaining liquid over the curds through the strainer. Do this slowly so the sieve does not overflow.
Leave it to strain for at least 6 hours or up to 12.
This can be done overnight in the fridge.
Once strained, add the quark plus enough of the strained liquid to total 600 grams into a blender. Blend on low speed and then gradually increase to high.