Wash and cube the chicken breast. In a pot, combine chicken, butter, salt, and white pepper. Shallow-poach on medium-high heat for 6-8 minutes until cooked through. Set aside. Slice onions, garlic, and leeks—heat oil in a pan over medium-low heat. Sauté onions for 6 minutes, then add garlic and leeks for an additional 2 minutes. Combine with the chicken in the same pan.
Clean and slice the mushrooms. Sauté in the pan for 6 minutes, then mix with the chicken and vegetables. Boil water and cook penne pasta for 8 minutes until al dente.
Add fresh cream and paprika to the chicken and mushroom mixture. Heat gently and mix—reserve 1/4 cup of pasta water. Combine the cooked penne with the sauce, adding the reserved pasta water to achieve a smooth consistency. Serve with grated Parmesan cheese, sliced parsley, and optional crispy croutons.