The only significant change I made to this recipe was to use goat cheese and mascarpone instead of ricotta, as that's what I had on hand. But it was delicious and I would do it that way again. Moreover, when I make this the next time, I will make two more changes: I would add some grated Parmesan cheese to the polenta to enhance the flavor, and most importantly, I would cut the total amount of polenta in HALF! The ratio of toppings to polenta would be much better that way, IMHO.
8 cups water (I would cut this to 4 cups)
1 3/4 teaspoons kosher salt
4 tablespoons butter
2 1/2 tablespoons olive oil, divided
1 large onion, sliced
2 cloves garlic, minced
16 ounces cremini mushrooms, sliced
12 ounces baby spinach or other greens (I used a baby spinach/baby kale mix)
6 oz. log of goat cheese
4 oz. mascarpone
2 teaspoons fresh thyme leaves (or 1 teaspoon dried)
Preheat oven to 375. In a heavy pot, bring six cups (I would just use three cups) water and salt to a boil. Gradually stir in polenta, stirring quickly to avoid lumps. Turn heat to low. When water is completely absorbed, add another half cup of water and continue to stir occasionally. Continue adding water, a half cup at a time, after each time water is absorbed by polenta. Continue to cook for at least 30 minutes, stirring occasionally. Stir in butter last (and Parmesan, if using).
In a wide saute pan over medium heat, heat olive oil. Add the onions and stir to coat the onions in oil. Season with salt and pepper to taste. Continue to cook over medium low heat until onions are golden and caramelized, stirring occasionally, about 20 minutes. Transfer onions to a bowl and set aside.
Heat remaining tablespoon of oil in the same pan. Add the mushrooms and garlic and cook until mushrooms release their liquid and then evaporates. (Also add thyme here if using dried.) Add greens and stir together until just wilted. Return onions to pan and stir to combine.
Spread the polenta on the bottom of a lightly oiled 9 x 13 baking dish. Arrange mushrooms, greens, and onions over the polenta. Mix the goat cheese and mascarpone together, then drop dolloped mounds of the blended cheeses over the vegetable mixture.
Bake for 20 minutes until cheese is melted and soft. Sprinkle with fresh thyme leaves to serve.