Mix bread with butter into a smooth mass and press it on the bottom of a springform (24 cm diameter).
Mix gelatine powder with water in a pot and allow to swell for a few minutes.
Mix crème frâiche, sour cream, red onion, dill and caviar in a bowl. Save some of the caviar as topping (approximately 40 g).
Melt the gelatine on low heat and stir it into the crème frâiche mixture.
Pour the mixture over the bread in the springform. Place in refrigerator overnight.
Before serving, cut in small pieces and sprinkle some caviar and dill on top.