Combine the arugula, radicchio, and fennel in a large bowl. In a smaller bowl, whisk together the lemon juice and olive oil in a bowl and season with salt and pepper. Taste the vinaigrette – feel free to add more lemon juice if you want a more acidic vinaigrette.
Toss the vinaigrette with the salad greens until lightly coated. Sprinkle with chopped walnuts and freshly grated Parmesan if desired.
VARIATIONS: This salad is delicious with sliced oranges and toasted hazelnuts (in lieu of walnuts). Try substituting ricotta salata for Parmesan. If you’d like to make this a main dish salad, it is delicious with grilled chicken, shrimp, or salmon. I also love serving this salad as a palate cleanser after a hearty pasta dish or stew.