Heat oil or butter in a heavy-bottom skillet over medium-high heat. While the pan is heating, apply kosher salt evenly to both sides of the chicken breasts. Once the pan is hot, add the chicken breasts and sear for 8–10 minutes a side until browned on both sides and internal temp reaches 165⁰F. Remove chicken from pan and set aside to rest for 5 - 10 minutes. Slice or chop the chicken right before adding back to the sauce (step 6 below).
Add diced onion and a small sprinkle of kosher salt, and sauté for approximately 5 minutes until translucent. Begin to scrape the bottom of the pan as the onion begins to release it's liquid.
Add garlic and cook until fragrant, about 30-seconds.
Add Italian seasoning and sundried tomatoes with some of their packing oil and sauté for an additional 1 to 2 minutes.
Add the heavy cream and chicken stock and deglaze the pan. Bring to a boil, and reduce heat to low and simmer for 8 to 10 minutes. The sauce should thicken enough to coat the back of a spoon.
Add chopped, fresh spinach to the sauce and sauté for an additional 1 to 2 minutes until wilted.
Add sliced or chopped chicken and plate juices back to the pan and stir.
Lower the heat and whisk in the parmesan cheese, a handful at a time until smooth.
Squeeze lemon wedges into the sauce and stir.
Adjust salt and pepper to taste.
Garnish with more chopped spinach and parmesan.
While the sauce is cooking, heat salted water until boiling and cook pasta according to package directions.
Add cooked pasta (including a small splash of pasta water) to the cream sauce and chicken mixture, and toss to coat noodles.
Notes
Add a splash of milk or heavy cream to the sauce before reheating the leftovers. This helps to keep the sauce close to its original consistency.