1. Preheat oven to 400°f. Coat a large baking dish with cooking spray and set aside.
2. In a large heavy-bottomed pot, over medium-high heat, add 1 tablespoon butter and olive oil. Add mushrooms; cook and stir until liquid evaporates, about 15 minutes.
3. Add garlic, onion and thyme; sauté until onion is tender, about 5 minutes. Stir in sherry and simmer for 2 minutes.
4. Remove from heat; add mascarpone cheese and stir until blended. Add prepared chicken and stir to combine. Cover and set aside.
5. Prepare linguine al dente according to package directions; drain and set aside.
6. In a small bowl, add chicken base with water whisk until base is dissolved; set aside.
7. In a large skillet, over medium heat, melt 3 tablespoons butter. Add flour, whisking continuously for 1-2 minutes. Whisk in water-base mixture, half-and-half, heavy cream, pepper and nutmeg. Bring to a gentle boil and then reduce heat to medium-low. Simmer and stir until sauce slightly thickens, about 10 minutes. Add sauce to pot of chicken mixture and stir to combine.
8. Add cooked linguine and peas to pot; stir to combine. Place in prepared baking dish.
9. Melt 4 remaining tablespoons of butter in microwave safe bowl; add breadcrumbs and parmesan cheese; stir to combine. Evenly sprinkle on top of chicken mixture.
10. Place baking dish on a rimmed baking sheet. Bake uncovered for 30-40 minutes or until heated through. If topping starts to over brown, lightly cover with foil.