Spray a saucepan with low-calorie cooking spray and place over a medium heat.
Add the onions and gently fry for 5-6 minutes until beginning to soften. Add the garlic and oregano and cook for a further minute.
Add the tomatoes and the stock and bring to the boil. Reduce the heat to a simmer and cover. Allow to cook for 20 minutes.
After 20 minutes, remove from the heat. Using a stick blender (or food processor) carefully blitz. We like to leave a little texture in the soup, but you can blitz it smooth if you prefer.
Return to the heat and bring back to a simmer. Add the orzo and cook for 7-9 minutes, stirring occasionally to prevent sticking.
When the orzo is cooked, stir in the balsamic vinegar and feta, reserving a little feta to garnish if you wish. You can add a splash of water to thin the soup down, if you think it is a little thick.
Taste and season with salt and pepper, if required. Serve into bowls and sprinkle on any reserved feta, if using.