Grilled Sardines and Okra with Black Garlic Aioli

Name: Grilled Sardines and Okra with Black Garlic Aioli
Ingredients: 12 fresh sardines, butterflied; 1/2 small onion, thinly sliced; 1/2 small fennel bulb, thinly sliced; 1 carrot, thinly sliced; 1 celery rib, thinly sliced; 10 thyme sprigs; 1/2 cup verjus (see Note); 1/4 cup rice vinegar; 1/4 cup plus 2 tablespoons extra-virgin olive oil; Salt and freshly ground pepper; 1/4 cup canola mayonnaise; 1 tablespoon lemon juice; 1/2 tablespoon black garlic (see Note); 1/2 teaspoon harissa; 1 bunch watercress, torn; 1 pound okra; 1 pint grape tomatoes