In a large bowl, whisk together the first 8 ingredients. Whisk in the beer until the batter is smooth, without lumps.
In deep pot, heat oil to 350F and watch thermometer throughout frying process, turning up the heat between batches so you can maintain the 350F while frying
Dredge pieces of fish into cornstarch and shake off excess. Dip a few pieces of fish into batter, let excess drip off, and gently submerse in the oil (I use tongs and a splatter guard). Do not overcrowd, working in small batches. Turn fish over in oil as needed to evenly brown all sides, until golden brown is reached. Drain on rack.
Eat while hot, and enjoy! If you happen to have any left over, use it for excellent fish tacos the next day.