Toast the walnut pieces over med-high heat in a dry skillet until aromatic and lightly browned. Let cool to room temperature.
Combine apple, onion, chives, parsley and toss well.
Pour vinegar into a small bowl, and whisk as you drizzle the olive oil into it to emulsify. Add the dressing to the apple/onion mixture (to taste) until well coated. Season to taste. The mixture can be stored overnight, if desired.
Just before serving, gently fold the walnuts and blue cheese into the apple & onion mixture (be very gentle and don’t allow the blue cheese to get creamy – keep it cold until just before you need it).
Separate individual spears from each endive. Spoon a portion of the salad into each spear, and serve!