1. For polenta, in a medium saucepan, bring the 2 1/2 cups water to boiling. Meanwhile, in a small bowl, combine cornmeal, the 1 1/2 cups water, and the 1 teaspoon salt. Slowly add cornmeal mixture to the boiling water, stirring constantly. Cook and stir until mixture returns to boiling. Reduce heat to low. Cook, uncovered, for 10 to 15 minutes or until very thick, stirring frequently. Spread evenly in an ungreased 3-quart rectangular baking dish; cool. Cover and chill about 30 minutes or until firm.
2. Preheat oven to 350 degrees F. In a large nonstick skillet, cook onion in hot oil over medium heat for 3 to 4 minutes or until tender. Add mushrooms, the 1/4 teaspoon salt, and black pepper. Cook and stir about 5 minutes or until mushrooms are tender. Remove from heat. Stir in roasted peppers.
3. Spread marinara sauce over chilled polenta. Top with vegetable mixture; sprinkle with cheese.
4. Bake, covered, for 30 minutes. Bake, uncovered, for 10 to 15 minutes more or until edges are bubbly. If desired, sprinkle with parsley.