If you're using homemade polenta, cook it until thick, pour it into a dish, and let it chill in the fridge for at least two hours until firm. Once it's set, slice it into half-inch rounds or sticks. If you're using a store-bought tube, just slice it straight from the package, no chilling needed.
Pour the oil into a large non-stick skillet and set it over medium heat. I let it heat until it shimmers, then test it with a small polenta crumb. If it sizzles right away, the oil is ready for frying.
Lay the slices in the pan in a single layer, making sure they're not touching. I fry each side for about 3 to 4 minutes until it turns deep golden and crisp. Crowding the pan is the one mistake that ruins the crunch.
Move the fried pieces onto a paper towel-lined plate right away. While they're still hot, I sprinkle on salt and pepper so it sticks to the crispy surface evenly.
Warm your marinara sauce in a small saucepan on low heat. I like plating the polenta with the sauce on the side for dipping, though spooning it right over the top works just as well.