1. Heat oil in large skillet over medium heat. Add onion and cook, stirring, until softened, about 5 minutes. Stir in collard greens, in batches if necessary, until they wilt slightly and all fit in the skillet. Pour in water and cook, stirring occasionally, until bright green and tender, 5 to 7 minutes, regulating heat to prevent burning. Drain.
2. For the dressing: Meanwhile, whisk vinegar, honey, ginger and butter together in a serving bowl. Add greens to bowl and toss with dressing. Season to taste with salt and pepper; serve warm.