This is a classic French bistro dish, perfect for brunch. The eggs are poached until the whites are firm. The yolks are just barely cooked so that, when broken, they run over the escarole leaves and combine with the vinaigrette. Frisee, the curly-leafed relative of escarole, may be substituted for the escarole. Or you can use a mixture of the two. For a bit of spice, add some arugula.