Fried Smelts with Cucumber Salad

Steps

1. Start with the cucumber. Halve it lengthwise, scoop out the seeds with a teaspoon if it is large, then cut it into 1/4-inch (5 mm) dice. Put it in a colander, toss with 1/2 teaspoon of the salt, and let it drain for 15 minutes over a bowl.
2. Dice the red onion to the same size and soak it in cold water for 10 minutes: it stays crunchy but loses its harsh bite.
3. Rinse the smelts in cold water. If they are under 4 inches (10 cm), leave them whole and ungutted: the innards are tiny and you eat everything, heads and bones included. Over 5 inches (12 cm), gut them: slit the belly with the tip of a paring knife or scissors, push out the innards with your thumb, and rinse. Then spread them on a clean kitchen towel and dry them very thoroughly, top and bottom: surface water makes the oil spatter and turns the coating soft.
4. Finish the salad before frying. Gently press the cucumber in the colander, drain the onion, and pat it dry. Chop the dill and slice the mint leaves. In a bowl, toss the cucumber, onion, herbs, the juice of one lemon, the olive oil, and the pepper. Taste: it should be bright and barely salty. Cut the second lemon in half.
5. Pour the sunflower oil into a large heavy saucepan or Dutch oven: it should come no more than a third of the way up. Heat it to 350°F (180°C), checking with a thermometer. Set a wire rack over a sheet pan, or line a sheet pan with paper towels, next to the stove.
6. In a large bowl, mix the flour, cornstarch, 1 teaspoon of the salt, and 1 teaspoon of the Espelette pepper. Right before each batch, toss a handful of smelts in the mixture, then shake them in a strainer so only a thin, even coat remains.
7. Fry the smelts in small batches, dropping them in one by one so they don't stick together. Fry for 2 to 3 minutes, stirring with a spider, until golden and the vigorous bubbling calms down: that means almost no water is left in the coating. Let the oil come back to 350°F (180°C) between batches.
8. Lift them out with the spider, drain on the rack, and salt them right away with the rest of the fine salt while they are still glossy with oil: the salt sticks instead of sliding off.
9. Spread the cucumber salad on a serving platter, pile the fried smelts on top, and set the lemon halves alongside. Pour the remaining Espelette pepper into a small dish so everyone can sprinkle it to taste. Serve right away: the crust softens within 10 minutes.