Savory Winter Greens Beignets

INGREDIENTS

• For Batter:
• 1 cup whole milk, warmed to 105°
• 1 teaspoon sugar
• 1 teaspoon active dry yeast
• 1 1/2 cups all-purpose flour
• 1 teaspoon kosher salt
• 2 large egg whites, room temperature
• For Filling:
• 6 tablespoons olive oil
• 1 medium white onion (about 8 ounces), halved lengthwise, thinly sliced
• 8 ounces assorted winter greens (about 12 cups), such as kale, mustard greens, mizuna, or arugula
• 1 cup (2 ounces) finely grated Parmesan
• 1 teaspoon kosher salt plus more for seasoning
• Vegetable oil (for frying)
• 1/4 cup vinegar powder*
• 1/2 cup all-purpose flour