Step 1

Bring milk and 3 cups water to a boil over medium-high heat in a medium saucepan. Reduce heat to medium. Whisking constantly, gradually add polenta; bring to a simmer. Reduce heat to low, cover pan, and cook, whisking every 10–15 minutes, until thickened and no longer gritty, 30–35 minutes. Remove from heat and add Parmesan, butter, and salt. Cook, whisking, until butter and cheese are melted and polenta is the consistency of porridge, about 1 minute.

Step 2

Do Ahead: Polenta can be made 3 hours ahead. Store covered at room temperature. Reheat over medium-low, adding milk as needed to loosen.