Ever eye those shrink-wrapped tubes of polenta in the grocery store and wonder what to do with them? Turns out they're the perfect base for casseroles — all you have to do is cut them into rounds first. The sauce, which I've been dreaming about for a while, is inspired by eggs Florentine. It's a basic Parmesan white sauce fragrant with garlic, onions, and, of course, a whole bag of sautéed baby spinach. Baked in a trusty casserole dish, this polenta Florentine is vegetarian comfort food at its finest.
Grab a bag of baby spinach the next time you're at the store, as that's the heart of the sauce. Sauté it in a dry frying pan first so that it wilts down, as this helps cook off the liquid so it doesn't dilute the rest of the sauce. The base of the sauce is a classic French béchamel (a white sauce made with flour, butter, and milk) enriched with Parmesan cheese, onions, and garlic. You can easily make this dish gluten-free by using cornstarch instead of all-purpose flour.
This casserole can be assembled in the time it takes for the oven to heat up. Cut the tube of polenta, make sure you season the rounds, and layer it in the baking dish while the sauce is cooking. Bake the casserole until bubbly and fragrant, then use the broiler to get some browning on top during the last few minutes. It makes a great main vegetarian dish or a side dish at your next dinner party.
To keep this dish truly vegetarian, look for Parmesan that is not prepared with animal rennet. The best way to tell? If you see "enzyme" in the ingredient list, it means animals were used to make the cheese (besides the milk).
Cooking spray or butter, for coating the baking dish
Freshly ground black pepper
1 (6-ounce) package baby spinach (about 6 loosely packed cups)
2 tablespoons unsalted butter
1/2 medium yellow onion, diced
1 clove garlic, minced
1/2 teaspoon kosher salt, plus more for seasoning
2 tablespoons all-purpose flour or cornstarch
2 cups whole milk
1/2 cup grated Parmesan cheese, divided
2 teaspoons freshly squeezed lemon juice
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.
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.
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.
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.
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.
- Storage: Leftovers can be stored in an airtight container in the refrigerator for up to 5 days.
- Make ahead: The spinach cream sauce can be made and refrigerated up to 2 days ahead. Bring back to a simmer before assembling the casserole.