Mediterranean Tuna and White Bean Salad with Pesto Vinaigrette

Name: Mediterranean Tuna and White Bean Salad with Pesto Vinaigrette
Ingredients: 2 (15-ounce) cans cannellini beans; 2 (5-ounce) cans oil-packed tuna; 1 medium lemon; 1/4 cup basil pesto ( homemade or store-bought); 1 tablespoon olive oil; 1 teaspoon Dijon mustard; 1/4 teaspoon kosher salt, plus more as needed; Freshly ground black pepper; 1 small fennel bulb (about 8 ounces); 1 small head radicchio (about 5 ounces); 2 ounces baby arugula (about 2 packed cups); Flaky sea salt, for serving (optional)