Sicilian Seafood Stew with Couscous

Name: Sicilian Seafood Stew with Couscous
Ingredients: 2 Tbs. extra-virgin olive oil; 2 celery stalks, diced; 1 fennel bulb, trimmed, cored and diced; 2 leeks, white and light green portions, thinly sliced and rinsed well; 4 garlic cloves, minced; 2 Tbs. tomato paste; 2 tsp. red pepper flakes; 1 cup (8 fl. oz./250 ml) dry white wine; 1 can (28 oz./875 g) diced tomatoes with juices; 4 cups (32 fl. oz./1 l) fish stock; 1 Tbs. saffron threads; 1/2 cup (2 oz./60 g) Castelvetrano olives, pitted and chopped; Kosher salt and freshly ground black pepper; 1/2 lb. (250 g) halibut, cut into 1 1/2-inch (4-cm) cubes; 1/2 lb. (250 g) clams, scrubbed; 1/2 lb. (250 g) mussels, scrubbed; 1/2 lb. (250 g) large shrimp, peeled and deveined; Cooked couscous for serving; Chopped fresh flat-leaf parsley for garnish; Lemon wedges for garnish