In a large nonstick skillet cook the onions in hot butter for 5 to 8 minutes or until tender, stirring occasionally. Add the wine, sugar, and salt. Bring to boiling; reduce heat. Boil gently, uncovered, about 15 minutes or until most of the liquid has evaporated and liquid is syrupy.
Meanwhile, place the spinach in a very large serving bowl. Spoon the onion mixture over spinach. Toss to combine. Season to taste with salt and pepper. Serve immediately.