Heat oil in a medium skillet over medium heat. Add onion and sugar; cook, stirring occasionally, until onion is golden and has caramelized, about 15 minutes. Add garlic; cook, stirring occasionally,
2 minutes. Add vinegar; cook until vinegar is warm, about 1 minute.
Add dandelion greens, and toss to combine. Cook until greens have just wilted, about 1 minute. Season with salt and pepper. Serve immediately.