Coat the bottom of the pan with cooking spray or a thin layer of oil and add the diced onion, minced garlic, sliced mushrooms and spinach.
Cook on medium-low heat until the spinach is wilted and the onions and mushrooms are softened, stirring occasionally.
While the vegetables are cooking, shred the cheese and set aside. In a separate bowl, beat or whisk the eggs until mixed well.
Add the eggs to the pan and begin pulling the cooked outer edges in towards the center of the eggs. Cook for about 3 minutes or until eggs are cooked through.
On a separate plate, set out 2 tortillas. Sprinkle half the cheese on top of the tortillas. (Your child can help with this step to get them excited about cooking and trying new foods!)
Divide the scrambled eggs/veggie mixture between the two tortillas, and top with the rest of the cheese and the sliced avocado. Close the quesadilla with the last 2 tortillas and put them in the pan to crisp each side, or enjoy them as-is!