Prepare the salad dressing by pouring the red wine vinegar, olive oil and salt and pepper into a sealable container.
Seal tightly and shake until the dressing is well blended.
Measure out two cups of spinach leaves, then roughly chop them on a cutting board. (Just run a sharp knife through the leaves a couple of times).
Place spinach in a medium sized bowl.
Add watermelon, onions and salad dressing and toss to coat.
Divide into serving bowls and top with walnuts and cheese, if desired.