Use a Dutch oven that holds 6 quarts or more. We like large yellow onions here, about 1 pound each, but sweet onions such as Vidalia or Walla Walla can be used. If using sweet onions, which are smaller and flatter, do not trim the ends (trimming will reduce the yield too much), and increase the salt to 2 teaspoons. We like an inexpensive lager for this recipe, but nonalcoholic beer works well too.

FOR THE SAUCE:  Whisk all ingredients in bowl until combined. Refrigerate until needed.

FOR THE ONION RINGS: Heat oil in large Dutch oven over medium-high heat to 380 degrees. While oil heats, combine sugar, 2 teaspoons cornstarch, salt, and onion powder in small bowl.

Adjust oven rack to middle position and heat oven to 200 degrees. Set wire rack in rimmed baking sheet and line with triple layer of paper towels. With sharp knife, remove ½-inch slice from each end of 1 onion and save for other use. Cut onion crosswise into ½-inch-thick rounds. Repeat with remaining onion. Separate onion rounds into rings, saving 3 innermost layers of each round for other use. Transfer rings to bowl. Sprinkle with 1 tablespoon confectioners' sugar mixture and toss to coat.

Whisk flour, baking powder, remaining 3 tablespoons cornstarch, and remaining 1½ tablespoons confectioners' sugar mixture in medium bowl to combine. Whisk in beer until almost smooth (small lumps are OK). Add one-quarter of rings to batter and stir to coat. Using your fingers or tongs, transfer rings to oil one at a time until surface of oil is covered. Fry rings, turning occasionally and adjusting heat to keep oil temperature between 360 and 375, until deep golden brown and crisp, about 4 minutes. Transfer rings to prepared rack and place in oven. Return oil to 380 degrees and repeat with remaining onion rings and batter. Serve with sauce.