Blend apple, yogurt, coconut, 2 tablespoons chopped onion, green chile pepper, lemon juice, and ginger in a blender until smooth; transfer chutney to a serving bowl.
Heat oil in a skillet over medium-high heat. Add mustard seeds to hot oil and cook until seeds start to pop, 2 to 4 minutes. Add 1 teaspoon sliced onion and curry leaf to mustard seeds; fry until onion is golden brown, 5 to 10 minutes. Pour seasoned oil mixture over chutney; season with salt.