Combine crumbled crackers and butter to obtain a smooth dough.
Cover the bottom of a Springform mold with the crust. Set in the fridge
Stir together agar-agar and cream in a sauce pan. Bring to a boil but stir constantly for about a minute. Remove from heat.
In a bowl, combine Stilton and quark. Add the cream and stir again
Pour the wet mixture onto the cracker crust. Place cheesecake in the refrigerator for at least 2 hours to chill.
Wash the grapes and cut in half. Remove the seeds and set aside.
Before serving, remove cheesecake from Springform and layer the grapes in a circular pattern, round side down, fitting them as close together as possible. They should cover the whole surface.
Top with chopped walnuts.