Heat oil in a large pot over medium heat. Cook and stir zucchini, onion, and garlic in hot oil until moisture comes from the zucchini, about 10 minutes.
Pour chicken stock and milk into the pot; bring to a boil and reduce heat to medium-low. Stir ground sage, salt, and pepper into the stock mixture; cook at a simmer for 20 minutes. Add Brie cheese; cook and stir until melted completely into the liquid, 5 to 7 minutes.
Pour soup into a blender no more than half full. Cover and hold lid down; pulse a few times before leaving on to blend. Pour blended soup through a strainer into a clean pot. Puree in batches until smooth.
Stir cream into the blended soup and place pot over medium-low heat; cook until warmed through, about 5 minutes. Garnish individual servings with fresh sage leaves.