Wash greens well and remove and discard stems. Chop leaves. In large pan or a Dutch oven, bring water to a boil and add the collard greens and onion. Bring water to a boil once more, then reduce to simmer. Simmer, uncovered, for 10 minutes. Drain well and return to pan. Stir in coconut milk, salt and pepper. Cook, uncovered, over medium-low hear for 10 minutes or more, until sauce is slightly thickened. Stir in tomatoes and cook until heated through. Serve immediately.