Combine 1/2 cup apple cider vinegar, 1/2 cup water, 1 tablespoon sugar, 1/2 teaspoon salt and 1/2 teaspoon peppercorns (if using) in a small saucepan. Warm gently just until sugar and salt dissolve, about 1–2 minutes. Remove from heat and let cool slightly for 5 minutes.

Place thinly sliced red onion in a heatproof jar or bowl. Pour warm brine over onions, pressing them down so they're submerged. Let cool to room temperature, then cover and refrigerate at least 30 minutes. (For best flavor, chill 1–2 hours; onions will keep up to 2 weeks.)

Lemon Vinaigrette

Emulsify dressing

In a small bowl or jar combine 3 tablespoons fresh lemon juice, 1 teaspoon white wine vinegar, 1 teaspoon Dijon mustard, 1 teaspoon honey (or maple), 1/2 teaspoon minced garlic, 1/2 teaspoon fine sea salt and 1/4 teaspoon freshly ground black pepper. Whisk briefly to combine.

Slowly stream in 1/4 cup extra-virgin olive oil while whisking constantly (or put lid on jar and shake vigorously) until dressing is emulsified and slightly thickened. Taste and adjust seasoning with more salt, pepper or lemon juice if desired.

Salad Assembly

Toast hazelnuts

If hazelnuts are not toasted, heat a dry skillet over medium heat. Add 1/2 cup hazelnuts and toast, stirring frequently, until fragrant and lightly browned, about 4–6 minutes. Transfer to a cutting board and coarsely chop once cool enough to handle.

If using whole pecorino, grate 1/2 cup finely. If pre-grated, measure 1/2 cup. If you prefer shavings, use a vegetable peeler to make thin slices from a wedge.

In a large salad bowl, place 6 cups mixed salad greens. Add 1/2 cup pickled red onion (drained) and 1/2 cup toasted chopped hazelnuts.

Drizzle about half of the lemon vinaigrette over the greens (start with ~2 tablespoons), then toss gently but thoroughly with salad tongs until leaves are lightly coated. Add more vinaigrette to taste, tossing between additions so you don’t overdress—total dressing used will be roughly 2–3 tablespoons depending on preference.

Scatter the 1/2 cup grated or shaved pecorino romano over the tossed greens. Finish with a small sprinkle of lemon zest if using and a final crack of black pepper.

Serve immediately in individual bowls or on a platter so greens remain crisp.