Bring a large pot of water to a boil and cook the pasta according to the package instructions. Drain, rinse with cold water, and set aside. Meanwhile, sprinkle the chicken with Old Bay Seasoning and fry it in olive oil, on medium heat, for 4-5 minutes on each side, until just cooked-through.
Remove the chicken from the pan and set aside. To the same pan, add the butter and onion, and cook for 5 minutes, until the onion softens and starts to turn a golden color.
Add mushrooms and a little salt, and cook for another 5 minutes. After that, add the wine and simmer until it is reduced by half, about 4-5 minutes.
Add the mascarpone and the reserved chicken, stir, and keep cooking for another 1-2 minutes, until the cheese has melted and heated through. Taste the sauce, and add more salt, if necessary.
Add the spaghetti and basil, and mix everything well. Serve immediately. Sprinkle some Parmesan cheese on top, if desired.