Preheat oven to 350 degrees Fahrenheit.
Combine all crust ingredients in a bowl. Mix with the help of a fork.
Press the mixture into a tart pan, making sure to carefully press into all the fluted sides. Bake for 12 minutes and let cool.
In a bowl, combine quark cheese, sour cream, and vanilla pudding mix. Using a whisk, mix vigorously until smooth.
Cut marshmallows in half, place on a plate, and microwave for 10 seconds only. You want to make sure to remove them from the microwave as soon as they start puffing up. Working quickly, incorporate marshmallows into the batter. Keep whisking until smooth again.
Gently spread filling over cooled crust. Use an offset spatula to help you get to all the corners.
Wash and dry blueberries well. Arrange blueberries into filling, pressing them in lightly.
Chill for at least one hour to set.
Tart will keep in the refrigerator for four to five days. Yields one 12×8.5-inch tart or six mini tarts