Put a baking sheet in the oven and preheat the oven to 425 degrees F.
Slice the tube of polenta in half lengthwise, then slice each half in half lengthwise again. Slice each piece into 1/4-inch-thick slices to make triangular pieces with a rounded base. Add to a large bowl, drizzle with the olive oil and toss to coat.
Remove the baking sheet from the oven and spray with nonstick cooking spray. Transfer the polenta bites to the baking sheet and bake for 20 minutes. Flip each bite and bake until crispy but still slightly tender on the inside, an additional 10 to 15 minutes. Sprinkle with the Parmesan and bake until the cheese is melted and lightly browned, another 5 minutes.
Let cool slightly, transfer to platter and serve with a side of marinara for dipping.