Grate or puree the onion
Taste it and if you find the raw harshness too strong, pour in 2 cups of cold water, let it sit for one minute, then strain out the water, pressing on the onion puree to remove as much water as possible
Combine all the rest of the ingredients with the onion, transfer to a container and this should keep for 3-4 days.
Use as needed on a salad, making sure to mix it well because a puréed salad dressing like this is a bit more difficult to mix into the salad than a traditional vinaigrette