Melt butter in a large saucepan over medium-high heat. Add mushrooms, red pepper, onion and garlic; cook, stirring about 5 minutes until tender.
Sprinkle with flour and stir. Gradually whisk in milk then Worcestershire, salt and pepper. Stirring occasionally, bring to a boil.
Add noodles, reduce heat to a low boil. Stirring constantly and around edges of pan, cook until pasta is tender, about 7 minutes.
Stir in chunks of Boursin, then chicken, peas, and parsley, stirring until chicken is hot and Boursin is melted. Serve sprinkled with basil and Parmesan.