In a large, deep frypan over medium-high heat, melt the butter. Stir in the onion and garlic, and cook 2 minutes. Mix in the coconut milk, water, rice and chicken and bring to a boil. Reduce heat to low, cover and simmer 20 minutes, until rice is tender and chicken juices run clear.
Stir the spinach into the frypan and cook just until heated through. Sprinkle with coconut, and serve.