Grill corn in the husks, or roast in the oven; cut kernels from cob.
In a heavy 4-quart saucepan, heat oil over medium heat. Stir in garlic and onions; reduce heat to low and cook until onions are translucent, about 5 minutes.
Stir in water and milk and bring to a boil over medium-high heat. Slowly stir in cornmeal, whisking thoroughly. Reduce heat to low and simmer for 30 minutes, stirring often to prevent sticking and burning. Season with salt and pepper; simmer 15 minutes more.
When mixture is thick and the cornmeal is tender, stir in corn, sage and Parmesan cheese. Transfer to a large bowl to serve.