Preheat the oven to 425°F and line a small baking sheet or bread pan with parchment paper
Heat oil in skillet
Place onion in a food processor and pulse until finely chopped. Transfer to the large, oiled skillet. Add carrot and celery to the food processor, and pulse until chopped. Add spinach and pulse a few times more. Add to the skillet.
Place the skillet over medium heat and cook, stirring. Continue cooking until liquid evaporates and vegetables begin to brown, about 5 minutes. Transfer to a large bowl.
Add egg, ground meat, balsamic, salt and black pepper to the bowl, and mix gently with your hands. Scrape mixture into the bread pan or onto the baking sheet and form into a loaf approximately 4 inches wide and 10 inches long. Spread top of loaf with ketchup. Bake until cooked through, about 40 minutes. Cool 5 minutes before slicing.
Recipe by OhYaGood at http://www.ohyagood.com/meatloaf-vegged/