Make the salad dressing: Get out a blender and add all of the salad dressing ingredients to it. Then blend it on high for about a minute until the onion is completely pureed. Transfer the dressing to a jar and let it sit in your fridge until it is cool.
Assemble and toss the salad: Get out a large bowl and add the spinach, nuts, and cranberries to it. Then pour half of the salad dressing over everything and toss until every piece of spinach is coated in the dressing. Taste it and add more dressing if you feel it needs it.
Serve the sweet spinach salad in bowls along with some of the reserved dressing and enjoy!