Wash the sweet potato thoroughly, prick 6-8 times with a fork and place on a microwave-safe plate. Cook in the microwave for about 3 minutes on the highest setting, then turn once and place in the microwave again for 3 minutes.
In the meantime, wash the cucumber and grate it roughly. Add the quark to the grated cucumber and stir until creamy. Season with salt, pepper and mustard. If you like, you can add a small, pressed clove of garlic.