Leeks With Hazelnuts and Tarragon

Place a rack in middle of oven; preheat to 325°. Toast hazelnuts on a small rimmed baking sheet, tossing halfway through, until deep golden brown, 12–15 minutes. Let cool, then coarsely chop; set aside.

Bring a medium pot of heavily salted water to a boil. Add leeks, reduce heat, and simmer, uncovered, until tender but still bright green, 12–14 minutes. Transfer to a large bowl of ice water and let cool; keep water in pot boiling. Transfer leeks to a cutting board and pat dry; slice in half lengthwise.

Using a slotted spoon, gently lower egg into pot of boiling water; cook 10 minutes. Transfer to bowl of ice water and let sit until slightly warm, about 2 minutes. Gently crack egg all over and peel; tear into bite-size pieces.

Whisk together garlic, mustard, vinegar, cayenne, 1 tsp. black pepper, and 1 tsp. Diamond Crystal or ½ tsp. Morton kosher salt in a small bowl. Gradually stream in ¼ cup oil, whisking constantly until vinaigrette is emulsified. Stir in 1½ tsp. tarragon.

Arrange leeks, cut side up, in a single layer on a plate. Drizzle liberally with oil and spoon vinaigrette over. Top with egg, tarragon leaves, and reserved hazelnuts. Season with more black pepper.