Season the fish - Rub salt all over the tilapia, including inside the cavity. Score both sides of the fish to allow better seasoning penetration. Let sit for 10 minutes.
Fry the fish - Heat cooking oil in a large pan over medium-high heat. Carefully place the tilapia in the hot oil and fry until golden brown, about 5-7 minutes per side. Remove and place on a wire rack to drain excess oil.
Prepare the base - In a deep cooking pot, heat 1 tablespoon of oil. Sauté the onion, garlic, and ginger until fragrant and the onion becomes translucent, about 2-3 minutes.
Add coconut milk - Pour the coconut milk into the pot with the aromatics. Bring to a gentle boil, then reduce heat to medium-low. Cover and simmer for 5 minutes.
Cook the fish - Add the Thai chili peppers and carefully place the fried tilapia into the coconut milk mixture. Cover and simmer for 12 minutes, allowing the fish to absorb the flavors.
Add peppers and seasoning - Add the sliced long green peppers, fish sauce, and ground black pepper. Stir gently to avoid breaking the fish. Cook for an additional 3 minutes.
Finish with spinach - Add the fresh spinach to the pot. Cover and cook for just 1 minute until the spinach wilts but remains bright green.
Serve - Carefully transfer the fish and sauce to a serving dish. Serve hot with steamed rice.