Place the guava, orange juice, apple cider vinegar, garlic, and water in the blender and process until smooth. If the dressing is too thick add a little more water or orange juice. Season with salt and pepper.
Combine all the ingredients for the salad and toss with guava dressing. Sprinkle with the toasted pistachios. Season with salt and pepper to taste.
You can use mixed greens or kale instead of spinach for the salad. Pecans and cashews would also be a good substitutes for pistachios.