How to Make It
In a small bowl, whisk the crème fraîche with the lemon juice and chopped herbs and season with salt and pepper.
Squeeze small handfuls of the shredded potatoes over a colander to remove as much liquid as possible. Transfer the potatoes to a medium bowl and toss with the flour; season with salt and pepper.
In a very large nonstick skillet, melt the butter in the oil. Add rounded tablespoons of the potato mixture to the skillet, about 2 inches apart; you should have 16 mounds. Flatten each mound slightly into a cake that's 1/2 inch thick and 1 1/2 inches wide. Cook the cakes over moderate heat, turning once, until golden and crisp, about 6 minutes. Drain on paper towels.
Arrange the potato cakes on a platter. Top each cake with a small dollop of the herbed cream, a few pieces of sturgeon and a few radish sprouts and serve.