Pour 4 inches canola oil into an 8-quart Dutch oven and bring to 350˚ over medium-high heat.
While the oil comes to temperature, peel the onions and slice into 1/2-inch-thick rings.
In a large bowl, combine the flour, kosher salt, garlic powder and black pepper. Whisk in the ale until the ingredients are combined. The batter should be thick but slightly loose.
Place a wire rack on a sheet pan. Working in batches, dip the onion rings into the batter. Drain off any excess batter back into the bowl, then slowly lower the onion rings into the hot oil. Cook, flipping the onions every 30 seconds, until golden brown, 2 to 3 minutes. Using a spider, transfer the onions to the rack and lightly season with sea salt.