Trim the stem ends of the Brussels sprouts and remove any loose or yellow outer leaves. Cut larger sprouts in half lengthwise so pieces are roughly even; leave small ones whole.

Peel the sweet potato and cut into 1/4-inch thick matchsticks or thin batons about 2–3 inches long so they cook quickly and crisp evenly. Pat both vegetables very dry with paper towels to limit splatter and help batter adhere.

Set a wire rack over a sheet pan lined with paper towels to drain fried pieces.

Batter & Coating

In a medium bowl whisk together the all-purpose flour, cornstarch and baking powder until combined.

In a separate bowl whisk the egg with the cold water until slightly frothy. Add the egg-water mixture to the dry mix and stir briefly to form a slightly lumpy batter — it should be thin enough to coat but not runny. If too thick, add up to 1–2 tablespoons more cold water; if too thin, add a touch more flour.

Place the prepared Brussels sprouts and sweet potato batons in a large bowl and season lightly with the kosher salt and pepper. Toss to combine. Working in batches, dredge the vegetables in the batter so each piece is evenly coated; shake off excess.

Frying

Pour the neutral oil into a deep skillet or medium pot so it reaches about 1 to 1.5 inches up the sides. Heat the oil over medium-high until it reaches 350°F (175°C) on a thermometer. If you don't have a thermometer, test with a small drop of batter — it should sizzle and rise steadily.

Carefully add a single layer of battered Brussels and sweet potato pieces to the hot oil without overcrowding (about 6–8 pieces depending on pan size). Fry, turning occasionally, until golden brown and crisp: Brussels halves will take about 3–4 minutes, sweet potato sticks 2–3 minutes. Adjust heat to maintain 325–350°F and prevent burning.

Use a slotted spoon or spider to transfer fried pieces to the wire rack to drain. Immediately season lightly with a pinch of salt while still hot. Repeat with remaining vegetables and batter.

Sauce & Finish

While frying the last batch, whisk together the Kewpie mayonnaise, honey (or maple syrup), rice vinegar, soy sauce and sriracha (if using) in a small bowl until smooth and slightly loose. Taste and adjust sweetness or acidity as desired.

Place the drained fried Brussels and sweet potato in a large bowl. Spoon the Kewpie sauce over them (use more or less to taste) and gently toss to coat evenly. Alternatively, serve the sauce on the side for dipping.

Transfer to a serving platter and sprinkle the furikake over the top followed by the sliced scallions. Serve immediately with lime wedges for squeezing over, if desired.