In a food processor, add the spinach and pulse a few times until chopped very small
Add the processed spinach, drained garbanzo beans, and chopped onion into a large bowl
In the food processor (no need to rinse the bowl!), add the lime juice, cilantro, mustard, sugar, garlic, cumin, and oil and process until smooth, scraping down the sides of the bowl as needed
Pour the dressing on top of the spinach/garbanzo bean mixture and stir well
Add salt and pepper to taste
Refrigerate for about 10 minutes to let the flavors develop