1. Cut the top of two apples and set aside. Hollow out the apples with a spoon, leaving 1 cm of flesh inside.
2. Cut the other two apples into small slices. Place the apple slices in a bowl and mix with lemon juice.
3. Chop the endives, saving a few leaves for decoration. Crumble the blue cheese with your fingers.
4. In a bowl, gently mix the apple slices drained of excess moisture, chopped endives, sliced celery, and cheese. Season with salt and pepper.
5. Fill the hollowed-out apples with the mixture. Decorate with endive leaves.
6. Replace the apple top. Serve cold.