Tip:To adapt a slow-cooker recipe to a conventional oven, follow these guidelines: add more liquid, to accommodate for greater evaporation; bring the dish to a boil over high heat in a Dutch oven, then cover the pot and put in a 350°F oven. Plan on the dish taking roughly half the time to cook in the oven as it would in the slow cooker.
Grease the slow cooker with cooking spray. Place the lemongrass on a small piece of cheesecloth, form into a bundle, and secure tightly with kitchen twine. Add to a medium-size, heavy saucepan, along with the milk, coconut milk, sugar, and salt. Stir and bring to a strong simmer over medium-high heat (to prevent curdling, do not let boil).
Pour the hot mixture, including the lemongrass sachet, into the slow cooker. Add the rice and stir well. Cover and cook on low until the rice is completely tender, about 2 hours (you want there to be some liquid left). Remove from the heat, discard the lemongrass sachet, and stir in the zest, vanilla, and cardamom.
Let cool slightly, then pour into individual serving bowls, cover, and refrigerate until cold. If desired, garnish with additional zest before serving.