Spicy Fried Shrimp Roll with Cardamon Sauce

Ingredients

• 1 red onion (peeled and thinly sliced)
• ½ cup warm water
• ½ cup apple cider vinegar
• 1 tablespoon granulated sugar
• 1 teaspoon kosher salt (plus more as needed for seasoning, divided)
• ½ cup sour cream
• 2 tablespoons milk
• 1 teaspoon (plus 2 tablespoons) freshly squeezed lemon juice (divided)
• 3 tablespoons minced flat leaf parsley (divided)
• 2 tablespoons minced chives
• 1 teaspoon finely grated lemon zest
• 7 teaspoons minced fresh garlic (divided)
• freshly cracked black pepper (as needed for seasoning)
• 1 serrano chile (seeded and minced)
• 1 teaspoon ground cardamom
• ½ teaspoon ground cumin
• 1 tablespoon olive oil
• 1 large egg
• 1 cup panko bread crumbs
• 16 large shrimp (about 1 pound) peeled, tails removed
• vegetable oil (as needed for drying)
• 4 tablespoons melted butter
• 4 top split hot dog buns (see note)
• 1 Hass avocado (peeled and sliced)
• lemon wedges (as needed for serving)