Combine first 7 ingredients in a large saucepan. Gradually add water, stirring constantly with a whisk. Bring cornmeal mixture to a boil, and reduce heat to medium. Cook polenta, 15 minutes, stirring frequently. Stir in Parmesan cheese. Spoon polenta into a 10-inch springform pan coated with cooking spray, spreading evenly.
Let polenta cool completely until firm (about 4 hours at room temperature).
Preheat oven to 35
Combine first 5 ingredients in a large nonstick skillet, and stir well. Cook over medium-high heat 7 minutes or until vegetables are tender and liquid nearly evaporates. Spoon into a bowl, and stir in tomato, garlic powder, onion powder, artichokes, and spinach.
Combine remaining ingredients in a small bowl, and stir well.
Add to mushroom mixture. Stir well; set aside.
Spread Spinach Filling over Herbed Polenta. Top with tomato slices; sprinkle with mozzarella cheese.
Place pan on a baking sheet.
Bake, uncovered, at 350 for 1 hour or until set.
Let cool on a wire rack 10 minutes. Chill 2 hours or until set.
