In a 2-qt saucepan, whisk together the cornmeal, 3 cups water and salt. Place over medium-high heat, stirring, until mixture comes to a boil. Reduce heat to low and stir every 5 minutes, scraping the bottom of the pan to keep polenta from sticking. Cook for 25 minutes, until polenta is thick. Add Parmesan and stir until melted.
To serve, divide spinach between 4 bowls and top with polenta. Measure ¼ cup of spaghetti sauce over each bowl and top with mozzarella. The mozzarella should soften in the hot sauce. Top with basil.