Peel the sweet potatoes and cut into cubes. Peel the carrots and cut into thick slices. Roughly chop the onion. Cook everything in the vegetable stock for about 20 minutes.
Drain the broth and coarsely mash the vegetables. Stir in the ginger and horseradish, then add olive oil. Season everything with salt, pepper, chilli flakes and lemon juice.
Serve as an accompaniment to fish, meat or bratwurst.