To prepare the polenta: Place a piece of parchment paper on an 8x8-inch square baking pan and brush with olive oil including the sides. Combine the polenta, stock, water, sage and herbs in a medium saucepan and bring to a boil over medium heat. Once it boils, reduce the heat to medium-low and cook, stirring constantly with a wooden spoon, until the polenta starts to separate from the sides of the pan, about 10-15 minutes. Stir in the butter and cheese and cook for another minute until the butter and cheese have melted and been incorporated into the polenta. Transfer the cooked polenta to the prepared baking pan; flatten and smooth the top with a rubber spatula. Allow to cool and set; this will take about 30 minutes.

To prepare the polenta "fries": Unmold the polenta onto a cutting board. Slice into 4x1-inch pieces. Brush the tops with olive oil and broil all sides. Each side will take 3-4 minutes to get crisp and golden.

To turn the "fries" into croutons: Cool the "fries" for a couple of minutes before cutting into squares with a sharp knife. If you cut them right out of the broiler, the polenta won't be as easy to cut in clean cubes.

To assemble the salad: Arrange the spinach, feta cheese, onions, prosciutto and croutons on a plate. Drizzle with your favorite dressing.