Heat olive oil in a large stockpot over medium-high heat; fry onion, carrots, and celery in oil until tender, about 5 minutes. Pour vegetable stock into the stockpot; add potatoes, leeks, and garlic. Bring the stock to a boil, reduce heat to medium-low, and simmer until potatoes are fork-tender, about 15 minutes. Remove stockpot from heat. Stir heavy cream into the soup. Puree soup in a blender in batches until smooth; season with salt and pepper to serve.