Combine the water and milk in a large, heavy pot or dutch oven over medium-high heat. Bring to a boil. Whisk in the polenta and cook, stirring constantly, until thickened, about 30 minutes. Remove from heat and stir in 1/2 teaspoon of the minced garlic, the salt, oregano, and basil, and then pour into the prepared casserole. Drizzle the olive oil over the polenta and sprinkle with the remaining minced garlic. Spread the spinach over the polenta in an even layer, then top with the onions, olives, and tomatoes. Sprinkle the goat cheese, feta and mozzarella on top.