Warm the olive oil in a large pot over medium low heat.
Add the minced onion, and cook until lightly browned.
Add in the diced carrot, celery, broth and diced tomatoes. Increase the heat to bring the pot to a boil, then add in the orzo, spinach, salt and pepper.
Cover the pot and reduce the heat to lowly simmer for 15 minutes, or until the orzo is tender. The soup will thicken after it's done cooking, so try to remove from heat when it's still a tad too soupy.
Serve with fresh bread and enjoy!
Notes
*Note that this soup will thicken the longer you leave it. For leftovers, add in some extra vegetable broth as needed before reheating to achieve desired consistency.