Combine milk, water, butter, and salt in a saucepan over medium-high heat. Bring to a boil, stirring gently. Remove from heat. Pour in semolina flour in a steady stream, whisking constantly to prevent lumps.

Place the saucepan back over medium heat; whisk until porridge comes to a boil, about 2 minutes. Reduce heat to low, cover, and cook, stirring occasionally, until porridge thickens, about 20 minutes. Remove saucepan from heat; stir in sugar.

Let porridge stand for 3 to 5 minutes before serving.