Toast walnut pieces in a dry skillet over medium-high heat until aromatic and lightly browned, 3 to 5 minutes. Let cool to room temperature.
Combine apples, onion, chives, and parsley in a bowl and toss well.
Pour vinegar into a small bowl; drizzle and whisk olive oil into vinegar until emulsified. Pour dressing into apple mixture and toss to coat.
Gently fold walnuts and blue cheese into apple mixture just before serving. Spoon filling onto each endive leaf.
I can get between 4 and 6 usable spears off of each endive. This is probably the most expensive item. I typically use the rest of the endives in a salad later, chopping or slicing them and incorporating them with other greens.
The dressed apple mixture can be refrigerated overnight before adding walnuts and blue cheese if desired.
Keep the blue cheese cold just before stirring into apple mixture to keep the blue cheese from turning creamy.