1 (24-ounce) tube cooked polenta
1 tablespoon olive oil
Salt and freshly ground black pepper
4 ounces sharp cheddar or Monterey Jack cheese, or combination, grated or thinly sliced
1 cup marinara sauce
1/4 teaspoon red pepper flakes
Arrange a rack in the top third of the oven and heat to 450°F.
While the oven is heating, cut the polenta crosswise into 1/2-inch-thick rounds (you will have about 16 slices). Place the rounds in a single layer on a rimmed baking sheet. Brush the rounds with the olive oil and season generously with salt and pepper. Bake until the polenta is golden-brown on top, 20 to 30 minutes.
Flip half of the rounds over with a thin, flat spatula. Divide the cheese evenly among these rounds. Return the baking sheet to the oven and bake until the cheese melts, about 3 minutes. Meanwhile, warm the marinara sauce and pepper flakes in the microwave or on the stove.
Place the remaining polenta rounds, browned-side up, on top of the cheese-covered polenta rounds to form sandwiches. Let the polenta grilled cheeses cool on the baking sheet for a few minutes before serving. Serve with the spicy marinara sauce for dipping.