Make the tahini sauce: In a medium bowl, whisk together the tahini, garlic, lemon juice, salt, and ¼ cup cold water to make a thick and creamy sauce. Set aside.
To a large skillet over medium-low heat, add 3 tablespoons of olive oil; when the oil is hot, add the onions and cook, stirring occasionally, until wilted, 10–12 minutes.
Add 3 tablespoons cool water and cook, stirring occasionally, until the onions have completely softened but have not taken on any color, 8–10 minutes more. Remove from the heat and set aside to cool to room temperature.
Preheat the oven to 425°F.Meanwhile, in a large, shallow bowl, stir together the lemon juice and zest, and ½ teaspoon cumin; season to taste with salt and black pepper, then add the fish fillets and turn several times to coat. Set aside to marinate at room temperature for 10 minutes. (Don’t leave it for much longer than this, otherwise the fish will start to break down.)
To a medium skillet over medium-high heat, add the remaining oil. Sprinkle the flour over a large plate and, one at a time, lift the fish pieces from the marinade and lightly dredge in the flour. Add the fish to the skillet (without crowding the pan) and cook, turning once halfway through, just until both sides are golden, about 2 minutes per side. Transfer the fish to a 9-by-13-inch baking dish and set aside.
Stir the cooled onions and the chillies into the reserved tahini sauce, then pour the sauce over the fish, smoothing the surface with the back of a spoon. Sprinkle with the pine nuts, transfer to the oven, and roast until the fish is just cooked through, 6–8 minutes. Switch the oven to the grill setting and continue cooking until browned and bubbly, about 3 minutes more.
Sprinkle with parsley and sumac and serve warm or at room temperature, with lemon wedges on the side.