Cook pasta according to package directions. Drain, reserving 1/2 cup of the pasta water; set aside. Chop fennel into 1-inch pieces. Snap off and discard woody bases from asparagus. Cut asparagus into 1-inch pieces. In a large skillet cook and stir fennel and asparagus in hot butter and olive oil over medium heat for 5 to 7 minutes or until nearly tender. Add garlic and cook 1 minute more.
Add pasta, mascarpone cheese, and reserved pasta water to skillet. Stir to combine. Stir in mozzarella, Romano cheese, and peas. Season with salt and pepper. Transfer to serving dish; sprinkle with parsley.