Purée cilantro leaves, coconut, coconut milk, garlic, serrano chile, coriander, and cayenne pepper in food processor until mixture forms a smooth paste. Set aside.
Place carrots and sweet potato in large skillet, and cover with 1 cup water. Bring to a simmer over medium heat, and cook 2 minutes. Add cauliflower, cover skillet, and simmer 2 minutes. Add green beans, cover, and cook 2 minutes more, or until vegetables are just tender. Stir in cilantro-and-coconut paste, and season with salt and pepper, if desired. Cover, and keep warm.
Meanwhile, heat oil in small skillet over medium-high heat. Add mustard seeds, and cover. Cook 30 seconds, or until sizzling subsides, then stir in curry leaves, if using. Stir mustard seeds and curry leaves into vegetables. Serve hot or at room temperature.