Boil a large, salted pot of water and cook the pasta al dente according to package directions.
Meanwhile, cut the chicken breasts in half lengthwise so you have 4 thinner cutlets. Sprinkle them with the garlic powder and some salt & pepper.
Add the olive oil and 1 tablespoon of the butter to a skillet over medium-high heat. Let the pan heat up for a few minutes. Cook the chicken for 5-6 minutes/side or until golden and cooked through (165F). Transfer to a plate.
Reduce the heat to medium. To the same skillet, add the remaining butter, mushrooms, and onions. Sauté until the water from the mushrooms is released/cooked off and they're golden brown (about 5-7 minutes).
Stir in the garlic, Italian seasoning, and white wine. Cook until reduced by half (about 1-2 minutes).
Stir in the lemon juice and cream, and scrape up any browned bits from the bottom of the skillet.
Cut up the cooked chicken into bite-size pieces.
In a small bowl, mix the cornstarch with 2 teaspoons of cold water to make a slurry, and then add it to the skillet and give it a good stir.
Add the chicken back into the pan and cook for a few more minutes until the chicken is heated through and the sauce thickens up a bit.
Stir the parmesan into the sauce.
Add the drained pasta to the skillet and toss until coated (add splash of the pasta water to the sauce prior to draining it if it's become too thick). Serve immediately with extra salt & pepper if needed, and garnish with chopped parsley if desired.