Water or stock can be substituted for any or all of the milk. The more milk you use, the creamier the polenta will be.
Place a medium saucepan over low heat. Add the bacon, and cook until crisp and golden, about 8 minutes. Remove bacon from saucepan; transfer to a paper-towel-lined plate. Set aside.
Add chopped sage to saucepan, and cook in the bacon fat until fragrant, about 30 seconds. Add milk, and bring to a boil.
Add the polenta in a steady stream, whisking constantly until it is smooth and creamy, about 6 minutes. Whisk in the butter, and season with the salt and pepper. Transfer to a serving bowl, and crumble the reserved bacon on top. Heat the olive oil, in a small saute pan over medium heat. Add the whole sage leaves, and fry until crisp, about 30 seconds. Remove from the skillet, and scatter over the polenta.