Preheat a large pan over medium heat (I use my big soup pot so I can fit all the spinach in at once). Saute the garlic in the oil for about a minute or until fragrant.
Add the asparagus and water, cover, and bring to a boil. Reduce heat to medium or medium-high and let simmer for about 5 minutes, until the asparagus is bright green.
Add the spinach (in batches if necessary, covering the pan and letting the leaves wilt so there's room for more). Once all the spinach has wilted (about 3 minutes), cook uncovered for about 5 minutes.
Meanwhile, put the cashew, capers, salt, and pepper in a food processor, and blend until cashews are small, coarse crumbs. Scrape down the sides to make sure you get everything.
When the spinach is cooked, add the veggies to the food processor, making sure to include all the garlic and any remaining liquid. Puree until relatively smooth. Add the lemon juice and adjust the seasonings if necessary.
Transfer to a container with a lid and refrigerate for a few hours. Serve with crackers, pretzel sticks, crudites, bread, etc. The sky's the limit. I think it would even make a good sauce for pasta.