Make the dressing: In a small bowl, whisk together olive oil, wine vinegar and shallot. Season to taste with salt and pepper.
Cut endive halves crosswise into 1/2-inch-wide pieces. Put endive, watercress, walnuts and mint in a salad bowl. Quarter and core apple, then slice crosswise as thinly as possible. Add sliced apple to salad bowl.
Add enough dressing to coat salad nicely; you may not need it all. Toss well, taste and adjust seasoning. Serve immediately.