Baked Fish in Tahini Sauce (Siniyet samak bil tahineh)

Name: Baked Fish in Tahini Sauce (Siniyet samak bil tahineh)
Cuisine: Middle Eastern
Ingredients: 1⁄2 cup plus 2 Tbsp. (5¼ oz.) tahini; 2 medium garlic cloves, coarsely chopped (2 tsp.); 2 Tbsp. fresh lemon juice; 1⁄2 tsp. kosher salt; 1⁄4 cup plus 3 Tbsp. extra-virgin olive oil, divided; 2 medium onions, thinly sliced (3⅓ cups); Six 5¼-oz. cod, hake, or halibut fillets; 1 Tbsp. finely grated lemon zest; 1½ tsp. ground cumin, divided; Kosher salt and freshly ground black pepper; 1⁄4 cup plus 2 Tbsp. all-purpose flour; 2 medium-hot green chiles, such as serrano or jalapeño, stemmed, seeded, and thinly sliced (⅓ cup); 3 Tbsp. pine nuts; 1⁄2 cup coarsely chopped Italian parsley leaves; 1 tsp. ground sumac; Lemon wedges, to serve