Mix salt, pepper, sugar and mustard with the vinegar in a salad bowl. Beat in the olive oil and stir in the finely chopped onions. Let the dressing steep a little.
Then cut the endive salad into thin strips and add to the dressing. Quarter the apple, then cut it into slices and add it as well. Mix together and do not let stand long before serving. Finally, drizzle the pumpkin seed oil over it.