1. Heat a ten-inch skillet that has a tight fitting lid over medium heat. Add 1 Tbsp. oil to the pan, and when you can see it shimmering, add the onions. Toss to coat the onions, and sauté until soft and just coloring.
2. Add the dandelions and cook until just wilted. Season with salt and pepper.
3. Make a well in the center of the greens, and add the garlic. If there is no oil standing in the bottom of the pan, add a little to the garlic. Sauté until garlic is softening and aromatic. Toss to evenly incorporate the garlic into the onion and dandelion mass.
4. Sprinkle the sugar over the lot and toss. Cook a couple minutes to caramelize the sugar. Add the liquid at this point. Put the top on the pan and simmer until the greens are tender. Remove the top and cook off the remaining liquid.
5. Drizzle with the vinegar, toss to mix in evenly, and serve.