- 1 cup whole milk, warmed to 105°
- 1 teaspoon active dry yeast
- 1 1/2 cups all-purpose flour
- 2 large egg whites, room temperature
- 1 medium white onion (about 8 ounces), halved lengthwise, thinly sliced
- 8 ounces assorted baby greens (about 12 cups), such as mustard, mizuna, and arugula
- 1 cup (2 ounces) finely grated Parmesan
- 1 teaspoon kosher salt plus more for seasoning
- Vegetable oil (for frying)
- 1/2 cup all-purpose flour
Place warm milk in a large bowl. Add sugar and yeast; whisk to dissolve sugar. Cover with a warm, damp kitchen towel and let stand at room temperature for 1 hour. Add flour and salt; whisk until smooth. Whisk egg whites in a medium bowl until frothy, about 1 minute. Add to batter and whisk until smooth.
Scrape down sides of bowl, allowing batter to pool at bottom. Cover bowl with a damp towel. Let stand at room temperature until batter begins to bubble, at least 1 hour and up to 2 hours. Stir. DO AHEAD Can be made 8 hours ahead. Cover and chill. Let batter stand at room temperature for 1 hour before using.
Heat oil in a large skillet over medium-low heat. Add onion and cook, stirring often, until softened but not browned, about 10 minutes. Working in batches, add greens to skillet and toss until wilted. When all greens have been added, cook, stirring frequently, until tender and mixture is almost dry, 4-5 minutes. Spread greens on a parchment paper-lined baking sheet and let cool completely.
Finely chop greens; transfer to a medium bowl. Stir in Parmesan and season with salt. Roll the seasoned greens into small balls, about 2 teaspoons each. Place on a rimmed baking sheet. Cover and chill for 1 hour. Keep chilled.
Pour vegetable oil into a medium heavy pot to a depth of 2". Heat oil to 360°. Combine vinegar powder and 1 teaspoon salt in a medium bowl. Set aside. Place flour in a medium bowl. Working in batches, roll vegetable balls in flour to lightly coat, then drop into batter, turning to coat evenly. Using 2 small spoons, lift balls from batter, allowing excess batter to return to bowl. Working in batches, fry beignets until golden brown, turning occasionally, about 2-3 minutes per batch. Using a slotted spoon, transfer to a paper towel-lined plate to drain. Roll hot beignets in vinegar-powder mixture and serve immediately.