To prepare the polenta, preheat your oven to 350°F. Spray an 8 x 8 inch baking dish with nonstick cooking spray. Whisk together the water, polenta grits and salt in your prepared baking dish. Then transfer the dish to your preheated oven and bake until the polenta is firm and all the water is absorbed, about 40 minutes. Remove the baking dish from your oven and set it aside to cool to room temperature. Then cover the dish and transfer it to your refrigerator. Chill the polenta for at least 4 hours or up to overnight. Once the polenta is fully chilled, slice it into 1-inch cubes.
To prepare the salad, preheat your oven to 350°F. Line two rimmed baking sheets with parchment paper. Slice the beets into ¼ inch thick rounds. Then slice the rounds down the middle to create half moon shapes. You should have about 2 cups of sliced beets. Toss the sliced beets in a bowl with 1 tablespoon of olive oil and ¼ teaspoon each of salt and black pepper. Spread the beet slices out onto one of the parchment paper-lined baking sheets. In a separate bowl, toss together the cubed polenta with the remaining 1 tablespoon of olive oil and ¼ teaspoon of salt and black pepper. Spread the polenta cubes out onto the second parchment paper-lined baking sheet. Transfer both baking sheets to your oven and bake for 15 minutes. The beets should be slightly softened and the polenta should be starting to crisp around the edges. Remove both baking sheets from the oven and set aside.
Meanwhile, in a small bowl, whisk together all of the ingredients for the dressing until well combined.
Add the mixed greens to a large bowl and drizzle with the dressing, tossing to coat. Then add the mint leaves, pistachios, goat cheese, roasted beet slices and polenta croutons to the bowl, tossing gently to combine. Transfer the salad to serving bowls and enjoy immediately.