Peel, core, and coarsely chop 5 sweet apples such as Fuji (about 2 1/2 lbs. total). In a 5- to 6-quart pan over medium heat, melt 2 tablespoons butter. Add 1 cup thinly sliced well-rinsed leeks (tender white part only) and cook, stirring often, until limp, about 5 minutes. Add apples, 1/3 cup apple cider vinegar or lemon juice, and 1/3 cup apple juice. Cook, stirring often, until apples are soft when pierced and all the liquid is evaporated, about 20 minutes.
Spread about 1/3 cup apple mixture and 2 tablespoons crème fraîche or sour cream down center of each Basic Buckwheat Crêpe in pan (step 4 of crêpe recipe). Sprinkle with salt and pepper. Cook until bottom of crêpe is browned, about 1 minute, then fold two edges over filling, leaving a little exposed.