Wash collard greens well. Remove and discard stems; cut up leaves (should have about 14 cups).
In a large pan or Dutch oven, bring water to boiling. Add collard greens and onion. Return to boiling; reduce heat. Simmer, covered, for 10 minutes. Drain well and return to pan.
Stir in coconut milk, salt, and pepper. Cook, uncovered, over medium-low heat for 10 minutes more or until slightly thickened. Stir in tomatoes; heat through. Serve immediately. Makes 6 to 8 side-dish servings.