Melt 2 tablespoons butter in heavy large saucepan over medium-high heat. Add onion and 2 teaspoons lemon peel. Cover and cook 5 minutes. Add couscous; sauté 2 minutes. Add broth and squash; bring to boil. Reduce heat to medium. Cook, uncovered, until couscous and squash are tender and mixture is still moist, about 10 minutes. Mix in radicchio and cheese. Season to taste with salt and pepper.
Meanwhile, finely chop parsley in mini processor. Add 4 tablespoons butter and 1 teaspoon lemon peel; blend well. Season with salt and pepper.
Transfer couscous to bowl. Swirl half of parsley butter into top of couscous. Serve, passing remaining parsley butter.