Stir coconut milk, tomatoes with juices, onion, garlic, ginger, curry powder, cumin and salt in a 4-quart slow cooker.
Cover and cook on high for 2 1/2 to 3 hours until the onion is tender and the sauce is hot throughout.
During the final hour of sauce cooking, bring rice and 1 1/2 cups water to a boil in a covered saucepan, reduce to low and cook for 40 to 45 minutes, then rest covered for 10 minutes.
Nestle cod pieces into the hot sauce, cover and cook on high for 20 to 30 minutes until the fish reaches 145°F and flakes easily. Avoid stirring the pieces apart.
Fold spinach around the fish, cover for 3 to 5 minutes until wilted and stir lime juice gently into the sauce.
Divide rice among 4 bowls and lift fish, spinach and sauce over it with a broad spoon.