Preheat the oven to 400 degrees and line a sheet pan with parchment paper.
Arrange four tortillas across the baking sheet, taking care to cover completely. Cover with 2 cups of cheese, then shredded chicken, onions and then the remaining cheese. Top with the last tortillas and brush with vegetable oil.
Bake for 20-25 minutes until tortillas are golden.
Remove from oven and flip out onto a cutting board. Divide into 8 portions and top with cilantro.
Serve with sour cream, salsa and avocado. Pack the remaining quesadillas for weekday meals.