This recipe is loosely adapted from one in Janet Mendel's excellent My Kitchen in Spain. I've chosen to give conservative quantities for salt, vinegar and cumin. Taste carefully once the gazpacho is made and add more seasonings to your taste.
Add cucumber, onion, salt and cumin. Blend until smooth. With motor running, add olive oil in a slow stream. Add more water as needed for the desired consistency, which could be runny or fairly thick.