Prepare Herbed Polenta:
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 350°.
Prepare Spinach Filling:
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.
To complete recipe:
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. Cut into 8 wedges; serve with spaghetti sauce. Garnish with oregano sprigs, if desired.