Arrange greens on a large platter. Heat oil and garlic in a small pan over moderate heat. Simmer garlic in oil to infuse the flavor. Remove the garlic from the oil and transfer it to a small bowl. Wipe the pan and return to heat. Add balsamic vinegar. Raise heat to high and reduce vinegar by 1/2, 30 seconds. Stream oil into saucepan and whisk to combine with vinegar. Drizzle dressing over the salad and season the greens with salt and pepper.