Start off by making the dipping sauce. In a food processor, add all the ingredients and blend until smooth. Then place it in the refrigerator.
Next, make the wet batter. In a large bowl, add all the ingredients then combine with a whisk. Add the mineral water to insure the batter is nice and runny.
Next slice up your onions into 1/2 inch thick rings. Rinse the mushrooms and pat them really dry.
Add the mushrooms and onions to a bowl and coat them with 1/4 cup cornstarch. Make sure they are all coated well.
Now preheat your oil to 350 degrees and start by adding the mushrooms to the wet batter. Coat them well, and fry them, (coat and fry all the mushrooms and onion rings) for 5-7 minutes or until golden brown. Then, place in a cooling rack.
Now, fry the onions for 3-5 minutes or until golden brown. Let cool 3 minutes before serving. Serve along with the dipping sauce.
Notes
You will need to fry them in batches. About 5-6 mushrooms at a time. You want to dip in the mushrooms and onion rings in the batter one at a time. (Dip and fry, dip and fry)