In a small bowl, whisk together the 1½ tablespoons Sriracha and ½ cup mayonnaise. Cover and refrigerate until ready to serve.

Add about 1½ inches of canola oil to a large, heavy pot or Dutch oven. Heat the oil to 350°F. Line a baking sheet with foil and paper towels for draining the fried onion petals.

In a large mixing bowl, whisk together the 2 cups flour, 1 teaspoon cayenne, 2 teaspoons salt, and ¼ teaspoon pepper.

Peel and rinse the onions. Cut each onion into quarters and separate the layers into individual petals. Discard the small center pieces. If any petals are especially wide, cut them in half.

Remove ¼ cup of the seasoned flour mixture and place it in a separate shallow dish.

Toss the onion petals in the reserved dry flour, coating them lightly.

Pour the 12 ounces beer into the remaining seasoned flour mixture. Stir gently with a fork just until combined. The batter should remain slightly lumpy; do not overmix.

Working in small batches, dip the floured onion petals into the beer batter. Shake off any excess batter.

Carefully place the battered onion petals into the 350°F oil. Fry for about 2–3 minutes, or until golden brown and crispy. Do not overcrowd the pot.

Use tongs or a slotted spoon to transfer the onion petals to the prepared baking sheet. Allow excess oil to drain.

Repeat with the remaining onion petals, allowing the oil to return to 350°F between batches.

Serve immediately with the chilled Sriracha mayonnaise for dipping.