Place ¼ cup stock, spring onion and garlic in a medium saucepan. Cook over medium heat until the onion is tender, about 2 minutes.
Add remaining stock, the spinach and the oregano; bring to the boil. Reduce to a simmer. Cover; cook until spinach is tender, about 5 minutes.
Stir in rice, lemon zest, lemon juice and salt; return to a simmer. Whisk ½ cup of the hot liquid into the whole egg and egg whites in medium bowl. Whisking constantly, pour warmed egg mixture into the simmering soup. Serve.