Prepare polenta according to package directions. Cover and keep warm.
In a large skillet cook onion in hot oil over medium heat for 4 to 5 minutes or until tender, stirring occasionally. Add garlic; cook and stir for 30 seconds. Add broccoli rabe. Cook, covered, about 3 minutes or just until tender. (If using broccoli florets, cook and stir for 3 to 4 minutes or until crisp-tender.) Stir in roasted red peppers.
In a small bowl gradually stir broth into cornstarch; add to vegetable mixture. Cook and stir over medium heat until thickened and bubbly. Cook and stir for 2 minutes more.
To serve, spoon vegetable mixture over polenta. Sprinkle with toasted nuts.