Stir the sliced garlic, 1 tablespoon olive oil, and a pinch of sugar together in a small bowl, then microwave briefly for 45 seconds or until the garlic softens. This gentle head start is my favorite trick in this whole recipe, it mellows the garlic and protects it from scorching in the skillet, which is the number one way garlic green beans go wrong. The garlic and the oil stay together from here on out. You'll use the whole mixture, garlic slices and all.
Pour half of the garlic-oil mixture, softened garlic included, into a large skillet over medium heat.
Once it's warmed through, add the 8 ounces trimmed green beans and 2 tablespoons water, then cover. The water creates steam that cooks the beans quickly while the garlic oil coats them in flavor. You're looking for bright, vivid green after about 3 minutes.
Add the remaining garlic-oil mixture. All of the softened garlic goes in here too, cover, and cook aabout 3 minutes more, then uncover and stir occasionally. This second stage is where the garlic turns golden and the beans go from bright green to tender-crisp. Watch the garlic color: golden is perfect, deep brown means pull it off the heat now.
Take the skillet off the heat, then stir in 1 ½ teaspoon white wine vinegar, ½ teaspoon salt, and ¼ teaspoon black pepper. Adding the vinegar off the heat keeps its bright, tangy flavor intact instead of cooking it away.
Give everything a good toss so each bean gets a little of that tangy coating, and serve hot.