Wipe mushrooms clean with a damp paper towel or give them a quick rinse and pat dry thoroughly. Trim any tough stems; leave whole smaller mushrooms and halved or quarter larger ones so pieces are roughly uniform (about 1–1½ inches).

Season and dredge

Place the 0.5 cup seasoned dredge flour (from the 'Seasoned flour and coating' section) in a shallow bowl or plate. Toss the mushrooms lightly with a pinch of salt to make the dredge adhere. Dredge each mushroom in the seasoned flour so it is fully coated; shake off excess and transfer to a tray.

Make the batter

In a medium bowl, whisk together 1 cup all-purpose flour (batter flour), 0.25 cup cornstarch, 0.5 tsp baking powder, 1 tsp kosher salt and 0.5 tsp ground black pepper. In a separate small bowl lightly beat the egg. Pour the egg into the dry mixture and start adding cold sparkling water (and 0.25 cup beer, if using) a little at a time, whisking until you have a smooth, slightly thick batter that coats the back of a spoon. Keep the batter cold (set bowl over ice if your kitchen is warm) — cold batter makes a crisper crust.

Coat mushrooms in batter

Working in batches, dip the dredged mushrooms into the batter, allowing excess to drip off, then immediately proceed to fry. Do not let battered mushrooms sit long before frying; the batter should stay cold and bubbly.

Frying & finishing

Heat oil

Pour about 3–4 cups (or enough to give 2–3 inches of oil depth) of neutral oil into a deep skillet or Dutch oven. Heat over medium-high to reach 350–375°F (175–190°C). If you don't have a thermometer, test with a small drop of batter — it should sizzle and float immediately without burning.

Fry mushrooms

Carefully add battered mushrooms to the hot oil in a single layer without overcrowding (work in small batches). Fry for 2½–4 minutes, turning as needed, until golden brown and crisp. Smaller mushrooms will cook faster; keep an eye on color. Remove with a slotted spoon to a wire rack set over a sheet pan or paper towels to drain. While still hot, sprinkle with the finishing fine salt and chopped parsley, if using.

Keep warm and repeat

Keep cooked mushrooms warm in a 200°F (95°C) oven while you fry remaining batches. If the oil temperature drops too low between batches, increase heat briefly so the next batch fries crisply rather than becoming greasy.

Finish and serve

Arrange fried mushrooms on a platter, squeeze lemon wedges over them if desired, and serve immediately with the dipping sauce or your preferred condiments.

Dipping sauce (optional)

While mushrooms are frying, whisk together 1/2 cup mayonnaise, 1/4 cup sour cream, 1 tsp Dijon mustard, 1 tsp hot sauce, 1 tsp lemon juice, 1/2 tsp minced garlic, 1/8 tsp salt and 1/8 tsp black pepper in a small bowl until smooth. Taste and adjust seasoning. Chill until ready to serve.