Fill sink with water. Add spinach, removing any large stems if desired; pat down to remove any grit. Transfer leaves to large pot in batches. Place pot over high heat, cover and cook until spinach is just wilted, about 5 minutes. Drain, rinse under cold water and drain again. Squeeze to remove excess moisture. Coarsely chop spinach and set aside.
In medium saucepan, heat oil over medium-low heat. Add onion, cover and cook, stirring occasionally, until softened, about 5 minutes. In small bowl, whisk together milk, cornstarch, flour and salt. Increase heat to medium and gradually whisk milk mixture into onion. Cook, stirring constantly, until mixture is steaming hot and thick, 3 to 5 minutes. Remove from heat. Stir in Gorgonzola if desired, tomato and spinach. Cover to keep warm.
Polenta: Prepare polenta according to package directions, using 4 cups water and 1 1/2 teaspoons salt. Spoon polenta into shallow bowls. Top with spinach mixture. Sprinkle with pepper and serve.