2 1/2 teaspoonskosher salt, plus more for seasoning
2 large eggs
1/2 cupwhole milk
1 1/2 cups dry, fine, unseasoned breadcrumbs
1 medium yellow onion, sliced into 1/4- to 1/2-inch-thick rounds and separated into single rings
2 quarts (8 cups) vegetable oil
Line a large baking sheet with parchment paper. Set aside 2 additional sheets of parchment.
Combine the flour and 1 teaspoon of the salt in a wide, shallow dish; set aside. Combine the eggs and milk in another shallow dish and mix until the eggs are broken up; set aside. Mix the breadcrumbs and remaining 1 1/2 teaspoons salt together in a third shallow dish; set aside.
To bread, toss an onion ring in the flour mixture until evenly coated, then shake off the excess. Next, dip the ring in the egg mixture until coated all over, then let the excess drip off. Finally, toss the ring in the breadcrumb mixture, pressing to get the crumbs to adhere, until evenly coated all over. Set the breaded onion ring on the prepared baking sheet and repeat with the remaining onion rings, placing them in a single layer and making sure they’re not touching. When the baking sheet is full, cover the breaded onion rings with a new sheet of parchment paper. Continue breading and placing onion rings on the new sheet of parchment until full. Repeat with the third sheet of parchment until all of the onion rings are breaded; set aside.
Place the oil in a Dutch oven or a large, heavy-bottomed pot. Heat over medium-high heat until it reaches 375°F on a deep-frying/candy thermometer. Place a wire rack on a second baking sheet; set aside.
Working in batches (about 8 medium to large onion rings at a time) and using your hands, carefully drop the rings into the hot oil and fry until light golden brown, about 45 seconds. Using tongs, transfer the rings to the wire rack and season immediately with salt. Repeat with the remaining rings and serve immediately.