Step 1To make the vinaigrette, place the basil leaves, parsley leaves and balsamic vinegar in a blender and blend until the mixture is bright green. Add the mustard, 1 teaspoon salt and the white pepper and blend until combined. Add the oil and blend until the vinaigrette is emulsified. Set aside (it should be used the same day).
Step 2Prepare the fish by first removing the heads. Remove any large scales and slit open the fish along the belly. Rinse them under running water, removing the intestines with your finger. Lay the fish open flat and with the tip of a knife, lift up the head end of the backbone; hold the fish down while gently lifting out the backbone, along with any small bones. With scissors, snip the backbone at the tail end, leaving the tail intact.
Step 3Season the fish with salt and pepper. Brush them with olive oil and 1 tablespoon chopped parsley and cook them on a hot grill, skin side down, until the meat turns translucent, about 4 minutes.
Step 4In a bowl, mix the frisee with the sliced cherry tomatoes, 1 tablespoon chopped parsley, the chives, scallions, shallots and torn basil. Lightly dress with the vinaigrette just before serving. Place the salad on a plate, arrange two grilled sardines next to the salad and garnish with a frond of fennel. Serve immediately.