Heat oil in a large saucepan over medium-high heat.
Add onion; saut 5 minutes or until tender.
Add ginger and garlic; saut 2 minutes.
Add water and tomatoes. Bring to a boil; reduce heat, and simmer 5 minutes. Stir in sugar, chile paste, and salt.
Remove from heat; let stand 5 minutes.
Place one-half of tomato mixture in a blender; process until smooth.
Pour pureed tomato mixture into a large bowl. Repeat procedure with remaining tomato mixture. Return pureed mixture to pan. Stir in coconut milk; cook over medium heat 2 minutes or until thoroughly heated.