Heat 1/4 cup olive oil in a large skillet. Cook the onions with 1/2 teaspoon salt over medium-low heat for 25 to 30 minutes until deeply golden and soft.
Reserve one-quarter of the onions for garnish. Add the cumin, coriander, cinnamon, turmeric, and rice to the skillet and stir for 2 minutes.
Cook Rice and Fish:
Transfer the rice mixture to a saucepan, add the stock, and bring to a boil. Cover, reduce to low, and cook for 16 minutes. Rest off heat for 10 minutes, then fluff.
Heat the oven to 425 F (220 C). Brush fish with remaining oil and season with remaining salt, pepper, and juice from half a lemon. Bake for 10 to 14 minutes until just flaky.
Sauce and Serve:
Whisk tahini, cold water, lemon juice, garlic, and a pinch of salt until smooth. Toast the pine nuts in a dry skillet.
Spread the rice on a serving dish, top with fish, reserved onions, pine nuts, and parsley. Serve with tahini sauce and lemon wedges.
Notes
Caramelized onions supply the traditional brown color, so cook them slowly rather than burning them.
Choose a firm white fish that flakes in large moist pieces.