Step 1

Arrange a rack in the middle of the oven and heat to 400°F. Coat a 9x13-inch baking dish with cooking spray or butter. Season the tops of the polenta rounds lightly with salt and pepper. Lay the rounds in the baking dish in 3 rows, overlapping them slightly; set aside.

Step 2

Heat a large frying pan over medium heat. Add the spinach and toss until completely wilted, 3 to 4 minutes. Transfer to a medium bowl.

Step 3

Melt the butter in the same pan over medium heat. Add the onion, garlic, and 1/2 teaspoon salt. Cook until softened, about 5 minutes. Add the flour or cornstarch and cook, stirring constantly, for 1 minute. Add the milk and bring to a simmer, stirring frequently with a wooden spoon. Continue simmering, stirring constantly, until thickened to the consistency of heavy cream, about 2 minutes more.

Step 4

Remove from the heat and stir in 1/4 cup of the cheese, reserved spinach, and lemon juice. Taste and season with salt and pepper as needed.

Step 5

Pour the spinach mixture evenly over the polenta. Sprinkle with the remaining 1/4 cup cheese. Bake uncovered until bubbling around the edges and golden-brown, about 25 minutes. Turn the oven to broil and broil until the surface is golden-brown in spots, 3 to 5 minutes. Let cool 10 minutes before serving.