Soak sunflower seeds for at least 8 hours (or up to 24 hours to make it easier for the blender).
Grate radishes and squeeze as much liquid from them as possible using a strainer. Put the liquid in a blender.
Add the rinsed sunflower seeds, radish water, lemon juice and salt to a blender. Pulse a few times until you get half-smooth consistency with some sunflower pieces.
Transfer everything into a bigger bowl along with grated radishes, chive, and black pepper. Mix everything gently.