Cook spaghetti according to package directions. Drain and set aside. Meanwhile, in food processor, pulse basil, baby spinach, chopped walnuts, Parmesan cheese, garlic and 2 tbsp. warm water 2 min. or until smooth. Season with salt and black pepper, if desired.
In skillet coated with nonstick cooking spray over medium heat, cook onions and peppers 3 min. or until tender, stirring often. Add hollandaise mix and milk. Bring to boil; cook 1 min. more or until slightly thickened, stirring occasionally.
Add spinach mixture, chicken and cooked pasta; toss to coat. Cook 1 min. or until heated through.