Place the quinoa in a saucepan and add twice its volume of water. Bring to the boil, then reduce the heat and simmer for about 15 minutes, until the water has been absorbed. Cover and let it rest for 10 minutes. This can be done in advance.
In a large mixing bowl, combine the drained lentils with the cooked quinoa and toss gently.
Add the diced beetroot, radishes, sun-blushed tomatoes and red pepper. Mix everything together.
Pour in the olive oil, cider vinegar and soy sauce, then add the cumin seeds. Toss well to coat.
Stir in the horseradish and season with salt and pepper. Taste and adjust the seasoning, adding more horseradish or vinegar if needed.
If you have time, let the salad rest for about 10 minutes to allow the flavours to develop.
Just before serving, fold in the fresh herbs.
Serve at room temperature.