Pour vegetable oil into a deep, heavy-bottomed pot or deep fryer to a depth of about 2½ inches (or enough to fully submerge pieces). Heat oil over medium-high heat until it reaches 350°F (175°C). If you don't have a thermometer, test by dropping a small pinch of batter; it should sizzle and rise to the surface quickly.

Make batter

In a medium bowl whisk together 1.5 cups all-purpose flour, 1/4 cup cornstarch, 1 tsp baking powder, 1 tsp salt, 1/2 tsp black pepper, and 1/2 tsp garlic powder until blended. In a separate bowl lightly beat 1 large egg, then whisk in 1 cup cold sparkling water. Pour the wet ingredients into the dry and whisk gently until just combined — a few small lumps are fine. For a thinner batter, fold in up to 1/2 cup cold beer or additional sparkling water. Keep batter chilled until ready to fry (place bowl over another bowl with ice if needed).

Dust produce

Place 1/2 cup flour (for dusting) in a shallow bowl. Toss the prepared mushrooms and zucchini pieces briefly in the flour to lightly coat; shake off excess. This helps the batter adhere.

Optional breadcrumb finish

If you want an extra-crispy finish, after dipping battered pieces, lightly press them into 1/2 cup breadcrumbs or panko before frying. This step is optional and will change texture.

Fry in batches

Working in small batches (do not overcrowd the pot), dip floured vegetables into the chilled batter to fully coat, letting excess drip off for 3–5 seconds. Carefully lower into the hot oil. Fry until golden brown and crisp, about 3–4 minutes for mushrooms and 2.5–3.5 minutes for zucchini (zucchini may brown quicker). Maintain oil temperature between 325–350°F (160–175°C) — if it drops, allow it to come back up before adding the next batch.

Drain and season

Use a slotted spoon or spider to remove fried pieces and transfer to a wire rack set over a baking sheet or paper towels to drain. Immediately sprinkle with 1 tsp kosher salt (or to taste) while still hot so the seasoning adheres.

Keep warm

If making multiple batches, keep finished pieces warm in a 200°F (95°C) oven on the wire rack so they stay crisp until serving.

Produce

Prepare mushrooms

Clean 12 oz mushrooms by wiping with a damp paper towel; trim stems as needed. Leave whole or halve larger mushrooms so pieces are roughly uniform in size for even frying.

Prepare zucchini

Wash 2 cups sliced zucchini and pat dry thoroughly with paper towels. Cut into 1/4-inch rounds or half-moons. Excess surface moisture causes batter to slip, so ensure pieces are well dried before dusting and battering.

Coating & Seasoning

Flour dusting

As noted above, lightly dust each piece of produce in 1/2 cup flour to create a dry surface for batter to cling to. Shake off excess flour so the batter doesn't become gummy.

Final seasoning and garnish

After frying and draining, sprinkle the fried mushrooms and zucchini with kosher salt to taste, then garnish with 2 tbsp chopped fresh parsley. Serve with lemon wedges for squeezing over the hot fritters.

Dipping Sauce (optional)

Make sauce

In a small bowl combine 1/2 cup mayonnaise, 1/4 cup sour cream or plain yogurt, 1 tbsp lemon juice, 1 tsp Dijon mustard, 1/2 tsp hot sauce (optional), 1/4 tsp salt and 1/4 tsp black pepper. Whisk until smooth and refrigerate until serving. Adjust seasoning to taste.