Put the onions in a large bowl and cover with milk. Let soak for 30 minutes.
Meanwhile, using a fine-sieve over a large bowl, sift together the flour, baking powder, 2 teaspoons salt, chili powder, and cumin, if using. Stir in the beer until a smooth batter is formed. Set aside.
Drain the onions and transfer to a kitchen towel-lined baking sheet. Blot the onions with kitchen towels until dry.
In a large, wide heavy-bottomed pot, pour in the oil to a depth of about 5-cm. Place over medium heat and heat to 200C. (The oil must be heated to 200C. since the temperature drops oil to about 190 C. when the onions are added.)
Working in batches, transfer about 8 to 10 of the onions to the batter and mix to coat well. Using tongs, pull several of the onions out of the batter, and let any excess batter fall back into the bowl. Carefully add to the oil and fry, flipping once, until golden brown and crispy, about 3 1/2 minutes per batch. Using the tongs, transfer to a dry kitchen towel-lined baking sheet to drain. Sprinkle with salt to taste. Repeat with the remaining onions. Serve immediately.
Cook's Note: If you would like onion rings with a thicker, puffier crust, dredge the onions in flour before adding them to the batter.
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