Trim bottom ends of onions. Blanch unpeeled onions, in batches, in rapidly boiling water in a large saucepan 45 seconds. (It's important to blanch in batches so that the water remains at a boil.) Drain and peel onions; place in a large zip-top freezer bag, seal, and refrigerate overnight, if desired.
Combine cabbage and 2 tablespoons salt in a large bowl; let stand 30 minutes, tossing occasionally. Rinse thoroughly, and drain well.
Combine onions and oil in a shallow roasting pan or a large cast-iron skillet; toss to coat. Spread onions in a single layer. Roast at 450° for 25 minutes or until browned, stirring after 20 minutes.
Combine 1/4 cup each vinegar and maple syrup; add to roasted onions. Roast 5 more minutes or until slightly thickened and onions are glazed. Set aside.
Combine remaining 2 tablespoons vinegar, 2 tablespoons maple syrup, 1 cup chopped pecans, sour cream, and 1/2 teaspoon salt in a food processor or blender; process 1 to 2 minutes or until smooth. Cover and chill dressing overnight, if desired.
Toss together chopped apple, cabbage, and endive in a large bowl. Drizzle each serving with dressing, and top with roasted onions; sprinkle with remaining 1 cup pecans.
Make Ahead: Blanch and peel pearl onions a day ahead; store in refrigerator. Prepare dressing up to a day ahead, and store in refrigerator.