in a deep pot, saute garlic and onion in oil for 2-3 minutes. Add broth and bring to a boil. as you slice the potatoes, add them carefully to the broth, and cook 20 minutes, stirring occasionally. The potatoes will begin to break up and thicken the soup as it cooks.
stir in the spinach in bunches, adding another handful as it wilts into the soup pot. stir in the tomatoes and season with salt and pepper. Let cook for 2-3 minutes more until it is heated thru. Remove from the heat and serve.