Cut the fish into large chunks, including the head.
Coat the fish: Coat both sides of the fish with flour, then shake off any excess flour.
Fry the fish: Heat enough oil in a large pot to fully cover the fish, bringing it to 180°C (350°F). Add the fish chunks in batches, making sure not to overcrowd the pot. Deep-fry until golden brown and cooked through. Remove the fish from the oil and place it on a paper towel-lined plate to drain.
For the fish stock: Transfer the fried fish heads to a pot, add 6 cups of water, and bring to a boil. Reduce the heat to medium and simmer for 15 minutes. Strain the stock and measure out 5 cups to use later for cooking the rice.
Julienne the onions: Cut each onion in half and place the halves flat-side down on the cutting board. Starting at one end, slice the onion into long, thin strips.
Rinse and drain the rice: Place the rice in a strainer and rinse it thoroughly under cold running water, using your hands to agitate the rice, until the water runs clear.
Toast the pine nuts: Transfer 3/4 cup of the oil used to fry the fish to a stockpot and heat over medium heat. Add the pine nuts and toast until golden. Remove with a slotted spoon, then repeat with the almonds.
Add the onions to the same stockpot and cook for about 10 minutes, stirring every few minutes. Continue cooking and scraping the bottom of the pot until the onions turn dark brown without burning. This step is crucial for achieving the characteristic flavor and color of a traditional sayadieh.
Cook the rice: Add the drained rice and stir for 10 seconds. Stir in the fish stock, then season with salt, cumin powder, and white pepper. Gently stir with a fork to combine. Once the mixture comes to a boil, reduce the heat to low, cover the pot, and simmer for 25–30 minutes.
Let the fried fish cool slightly, then gently pull away the flesh with a fork or your fingers, following the bones. Remove the backbone and any small pin bones, checking carefully before serving.
Serve the sayadieh: Transfer the cooked rice to a platter.
Top with fried fish chunks and garnish with toasted pine nuts and almonds.