Neutral cooking oil, such as peanut, canola, or cottonseed, for frying
4 sweet onions, such as Vidalia or Walla Walla
1 cup all-purpose flour
1 (12-ounce) bottle lager-style beer, such as Budweiser
2 teaspoons cornstarch
1 teaspoon kosher salt
Freshly ground black pepper, to taste
Fill a heavy Dutch oven or cast iron pot with a few inches of oil and heat on medium-high until it reaches 365° to 375°F. Line a baking sheet with paper towels or a brown paper bag.
While the oil heats, slice off the root and tip ends of the onions. Peel the onions and cut them crosswise into 1/4-inch to 1/2-inch slices. Separate slices into individual rings, discarding the centers (or saving them for another purpose).
In a shallow mixing bowl, whisk the flour, cornstarch, salt and pepper to combine. Whisk in just enough beer to make a smooth paste, working the mixture as little as possible. Whisk in more beer (about 1/2 cup) until the batter is smooth and creamy, about the consistency of pancake batter.
Using tongs, dip an onion ring into the batter and flip to coat completely, shaking excess batter back into the bowl. Drop the ring into the oil and repeat, working in batches so as not to crowd the pot. (Allow the oil to come back up to temperature before continuing with another batch.)
Fry until golden, flipping occasionally, about 4 to 5 minutes. Remove the rings using a spider or tongs, and transfer to the prepared sheet pan to drain. Season with additional salt and serve immediately.