How to Make It
Whisk vinegar, lemon juice, salt and pepper in a large bowl. Slowly drizzle in vegetable and olive oils, whisking constantly. Add radishes and apple. Toss well to prevent discoloration. Cover and refrigerate until serving. (Salad may be made up to 3 hours ahead to this point.)
Just before serving, add endive and toss well to coat. Serve cold.