Wash and dry the spinach leaves thoroughly.
Peel the mandarins and separate them into segments.
Slice the avocado in half, remove the pit, and cut it into thin slices.
Thinly slice the red onion.
In a large bowl, combine the spinach, mandarin segments, avocado slices, and red onion.
In a small bowl, whisk together the balsamic vinegar and olive oil.
Drizzle the dressing over the salad ingredients.
Toss gently to combine, ensuring the avocado and mandarin segments remain intact.
Sprinkle chopped walnuts on top for added crunch.
Season with salt and pepper to taste before serving.