In the bowl of a food processor, combine the scallions, parsley, dill, mint, capers and garlic and process until finely chopped.
Add the avocado, lemon juice, vinegar and olive oil. Season with salt and a few grinds of black pepper. Process until smooth, thick and creamy. Taste for seasonings and add more salt and pepper to taste.
Place the chicken in a large bowl. Add about 1 cup of the dressing and toss well, adding more to taste. Serve immediately, sandwiched between slices of toast, with the rest of the dressing on the side, or cover, placing plastic wrap directly on the surface of the chicken salad to prevent browning, and refrigerate for up to 2 days before serving.
