Combine the flour, beer and 1/4 teaspoon salt, stirring well. Cover and let stand at room temperature 1 1/2 - 2 hours.
Peel the onions. Cut into 1/4 inch thick slices and separate into rings. Dip the rings into the beer mixture. Fry in deep hot oil (about 400ºF) until golden brown. Drain on paper towels, and sprinkle immediately with salt.
To eat them when they are crispy. Onion rings are best served immediately after frying.