Onion Rings from W S

This recipe arrived with my William Sonoma Magazine and I didn't see another quite like it - it looks so good I'm posting and planning on making these at my next opportunity. With the beer and mustard they sound differant from the other recipes I've seen here. Think they'll go great with a hot panini. YUM! Plan ahead as they need to be in the fridge overnight and this is not counted in prep time.

• Ready In: 30 mins
• Serves: 4-6

Ingredients

• 2 -3  vidalia onions, large and cut into 1-inch thick slices and separated into rings
• 4 -6  cups buttermilk
•  vegetable oil, for deep frying
• 1 1⁄2 cups all-purpose flour, up to 2 cups to make a thick batter
• 1⁄3 cup cornstarch
• 1 1⁄2 teaspoons kosher salt, plus more, to taste
• 3⁄4 teaspoon baking soda
• 2  eggs
• 2  tablespoons  whole grain mustard
• 2 2⁄3 cups dark beer

Directions

1. Divide onion rings between 2 large sealable plastic bags;.
2. add enough buttermilk to coat rings.
3. Seal bags; refrigerate overnight.
4. Set wire rack on baking sheet; place in oven.
5. Preheat oven to 200.
6. Line another baking wheet with paper towels.
7. Pour oil into deep fryer and heat to 350 according to manufacturer's instructions.
8. In large bowl, whisk flour, corn starch, 1 1/2 teaspoon salt and baking soda.
9. In another bowl, whisk eggs, mustard and beer.
10. Whisk egg mixture into flour mixture until well incorporated.
11. Drain onion rings.
12. Place handful of rings into batter; stir to coat thoroughly.
13. Remove 1 onion ring at a time from batter; let excess drip off.
14. Add 6-8 onion rings to hot oil; fry, turning occasionally, until golden brown, 3-5 minutes.
15. Transfer to paper towel-lined baking sheet; sprinkle with salt.
16. Place on rack-lined baking sheet in oven.
17. Repeat with remainion onion rings.