In a heavy saucepan, bring 4 cups of the broth or water to a boil over high heat. Add 1 teaspoon salt and and reduce the heat to the medium. Add the polenta gradually, whisking constantly. Continue to whisk for 5 minutes as the mixture bubbles. Reduce the heat to low and cook, stirring occasionally, until the mixture begins to come away from the sides of the pan, about 10 minutes. Add 1/2 cup of the Parmesan cheese and 1/2 teaspoon pepper and stir to combine thoroughly.
The polenta can be made into triangles and broiled up to 4 hours ahead of serving -- just reheat in a 350°F oven until warmed through.
Invert a rimmed baking sheet on a countertop and cover the surface with parchment paper. Spread the polenta out with a spatula into a smooth, even layer. Cover the polenta with another sheet of parchment paper. Place another baking sheet, right side up, on top of the parchment. With 2 hands, press down evenly on the top baking sheet until the polenta is 1/4 inch thick. The idea is to sandwich the polenta between 2 flat surfaces. Refrigerate for at least 1 hour or up to overnight. Meanwhile, make the harissa.
Turn a gas burner on medium heat and char the jalapeño, turning frequently, until blackened and slightly collapsed. Transfer the jalapeños to a cutting board to cool slightly. Once cool enough, trim off the stems from the jalapeños and remove the seeds and ribs.
If you like spice, add the seeds and ribs, or taste and adjust to your liking.
Combine the jalapeños with all the spinach, parsley, remaining 1/2 cup of the broth, walnuts, garlic, lemon juice, cumin, lemon zest, and 2 tablespoons of the olive oil in a food processor or blender and blend, scraping down the sides as needed, until smooth. Taste and add salt and pepper as desired. Transfer to a bowl and add remaining 2 tablespoons of olive oil and stir to help loosen the sauce.
The harissa be made up to 2 days ahead of time. Store refrigerated until ready to use.
When the polenta is cool, heat the oven to 375°F. Invert the baking sheets together. Remove the top baking sheet and the top layer of parchment paper. The smoother side of the polenta will be facing up. Cut the polenta into triangle about 2 inches long by 1 1/2 inches wide. Transfer the triangles to 1 or more baking sheets, spacing them 1/2-inch apart. Sprinkle about 1/2 cup of the remaining Parmesan cheese on top of triangles.
Bake the crostini until hot throughout and starting to brown, about 10 to 12 minutes. Turn the oven to broil and broil the crostini about 8 inches from the heat source until crispy and golden brown on top, about 3 minutes. Serve each polenta triangle with a dollop of the green harissa, a crumble of feta, and a drizzle of olive oil.