Pour 2½ cups water and milk into a medium saucepan and bring to a boil. Reduce the heat to medium and add the cornmeal in a slow stream, whisking constantly, avoiding lumps.
Reduce the heat to medium-low and continue cooking for about 10 minutes, whisking frequently, until the polenta is thick and it starts to pull away from the sides of the pan.
The consistency should be that of thick oatmeal; if it’s too thick, you can stir in a little water.
Stir one tablespoon oil into the cooked polenta, then spoon it out onto an oiled baking sheet. You’ll need to work fairly quickly as the polenta sets up as it cools.
Spread the polenta into a circle, about ½-inch thick. Sprinkle the round with salt and pepper and let it cool.
Cover the baking sheet with plastic wrap and place in the refrigerator to firm up, at least an hour but preferably overnight.
When you’re ready to bake the polenta, put it a 450° oven for about 25 minutes, until it begins to brown a little on the edges.
Meanwhile, heat two tablespoons of the oil in a large skillet over medium heat. Add the onion and bacon and cook until the bacon is browned and the onion is soft, about 10 minutes.
Using a slotted spoon, remove the onion and bacon to a bowl and set aside.
Cook the spinach in the same skillet until it releases its water and the pan becomes dry.
Season with salt and lots of freshly ground black pepper and set aside.
Remove the polenta from the oven, spread the grated cheese over the top, then spread the sautéed onions, bacon, and spinach on top of the cheese.
To serve, cut the polenta into four wedges and drizzle each portion with a little more olive oil and sprinkle with sea salt and freshly ground black pepper.