Bring 3 cups of vegetable broth to a boil in a sauce pot. Stir together the cornmeal and one cup of water, then pour into the boiling broth. Stir the mixture and allow it to come back up to a simmer. Turn the heat down to low, place a lid on top, and allow to simmer for 10-15 minutes, or until very thick. Stir occasionally to prevent it from sticking to the bottom of the pot.

Mince the garlic and slice the mushrooms. Add the olive oil, mushrooms, garlic, and thyme to a large skillet and sauté over medium heat until the mushrooms are limp. Season the mushrooms lightly with salt and pepper.

Remove the mushrooms from the skillet (leaving any liquids behind in the skillet) and add the frozen spinach. Sauté the spinach until heated through. Season lightly with salt and pepper. (It's okay if the mushrooms and spinach are a little on the bland side because the marinara, feta, and polenta all provide a lot of flavor and salt.)

Spoon one cup of polenta into each bowl, then top with 1/4 cup of marinara sauce (warm the marinara sauce in the microwave or a small pot if desired). Divide the mushrooms and spinach between the four bowls, then crumble 1/2 oz. of feta over each bowl. Serve warm.