Make the broth: Simmer broth, water, and smashed lemongrass for 10–15 minutes. Keep warm.
Cook the aromatics: Sauté onion (or shallots) and garlic in oil and butter until soft.
Toast the rice: Add the Arborio rice and stir for about 2 minutes. Add the wine and let it absorb.
Add broth slowly: Add warm broth one ladle at a time, stirring and letting it absorb before adding more.
Finish the risotto: When the rice is creamy and tender, stir in butter, Parmesan, and lemon zest.
Season and serve: Add salt and pepper to taste. Garnish with herbs and enjoy!