1Slice the onions crosswise about 1/4-inch thick, separating them into rings.
3In a medium bowl, whisk together the flour, cornstarch, baking powder and a pinch of salt. Whisk in the beer until smooth – it should have a texture somewhere between pancake batter and heavy cream.
5Heat about an inch of oil in a heavy medium pot set over medium-high heat. When it’s hot, but not smoking – a scrap of bread should sizzle when dipped in – dunk a few onion rings at a time in the batter to coat, then cook in the oil, turning with tongs as they turn golden. Transfer to a paper towel lined plate and shower with coarse salt. Serve warm - they're delish with lemon aioli for dipping. Serves 4-6.