All measurements are approximate and can be changed to suit individual tastes.
Put spinach into a container with a lid. I use an empty Soy Garden tub because it's easy to take in my school bag.
Dice the onion and sautee in a non-stick pan over low heat. While the onion is cooking, chop the asparagus into 2 inch pieces.
When the onion is translucent and begins to smell sweet, add the asparagus.
When the asparagus is bright green, remove from heat and add cooked veggies to the spinach.
Toast the walnuts in the pan used for the veggies. Watch carefully so they don't burn.
Add walnuts, chopped sundried tomatoes, and a drizzle of balsamic vinegar to the veggie mixture.
Cover the container and shake lightly to mix.
I usually put this into the fridge and leave it overnight. It can be eaten warm right away, but should be covered for a few minutes to let the spinach wilt. I also don't use oil to cook the veggies. If you are worried about burning them you can add a few tablespoons of water to the pan with the asparagus, but be sure to drain the pan before emptying it into the container with the spinach.
Variations: Try using raisins instead of sundried tomatoes or adding mushrooms. I don't recommend mushrooms AND raisins. Also try sprinkling with almond meal or sunflower seeds instead of toasted walnuts.