Cook the rice and quinoa (separately) according to package instructions. While the quinoa is in the last 3 minutes of cooking, add the peas to the pot and re-cover, so that they steam while the quinoa finishes cooking.
While the grains cook, blend all dressing ingredients together till smooth. The dressing should be creamy but pourable; if it's too thick, add an extra tablespoon or two of water to thin it out.
Fluff the grains and allow them to cool for 10 minutes. Transfer the grains and peas to a large mixing bowl and add the carrots and radish, along with fresh herbs if you like. Pour the dressing over and mix everything well. Taste and add salt and pepper as needed, along with an extra splash of vinegar if desired. Serve.