Prepare your pasta of choice and set aside. Make sure to reserve some of the pasta water in case you need to thin the sauce with it.
While the pasta cooks heat a large skillet over medium-heat high, add the bacon pieces, and cook until fat is rendered out, and bacon is cooked completely. Remove bacon and set aside, and leave bacon grease in the skillet.
Dry chicken off really well with paper towels. In a bowl, season the chicken fillets liberally with salt, pepper, and oregano on both sides, saving some seasonings for later.
In a hot skillet, melt 2 tablespoons of butter. Sear the chicken until it is browned on both sides and cooked through completely. A quick read thermometer should read 165°F. Remove chicken from the skillet, and leave browned bits in the bottom of the pan. Allow to rest for about 10 minutes. After 10 minutes, chop chicken into bite-sized pieces and set them aside.
Add 2 more tablespoons of butter to the skillet, followed by onions and mushrooms. Cook until beginning to soften, about 3-5 minutes, then season with a pinch of salt and remove from the skillet and set aside.
Add the remaining two tablespoons of butter, followed by garlic and oregano to the skillet. Cook until garlic is fragrant, about 30 seconds, then immediately pour in the white wine to deglaze the pan. Cook for 3-5 minutes to cook off the alcohol.
After the sauce has simmered for a few minutes, then add in heavy cream, half and half, lemon juice, and parmesan cheese. Stir continuously to melt the parmesan cheese. Once the sauce has formed, add the spinach to the cream sauce and allow it to wilt.
Add the onions, mushrooms, chicken, and bacon to the skillet to warm through and finish the sauce. Taste and adjust seasonings, add pasta water to thin the sauce if desired.
Serve over pasta. Garnish with parmesan cheese, fresh cracked black pepper, and optional minced parsley.
Store in the fridge in an airtight container for up to 5 days.