Bring the stock to the boil in a large saucepan. Set aside a few florets of broccoli to use as a garnish, then add the rest to the saucepan along with the onion and garlic. Return the stock to the boil, then tip the rice into the saucepan. Cover and cook over a low heat for 20 minutes, or until the broccoli and rice are tender.
Meanwhile, cook the reserved broccoli florets in a little lightly salted, boiling water for 5 minutes until just tender. Refresh in cold water and drain on paper towels.
Take the saucepan of stock off the heat and allow the mixture to cool slightly, then blend it in a blender or food processor until smooth. Return the soup to the pan to reheat. Stir in the sour cream and season to taste. Serve garnished with the reserved broccoli florets and the toasted almonds.