Boil penne in salted water until al dente. Reserve pasta water, then drain.
Cook bacon in a large skillet until crisp. Transfer to a plate; leave 1 tablespoon drippings.
Season chicken with salt, pepper, and Italian seasoning. Sear in olive oil and butter until cooked through.
Rest chicken 5 minutes, then slice thinly.
Sauté onion in the skillet over medium heat until softened. Stir in garlic and red pepper flakes.
Pour in chicken broth. Scrape browned bits and simmer to reduce slightly.
Stir in heavy cream. Simmer gently, then whisk in Parmesan until smooth.
Fold in spinach to wilt. Add lemon juice for brightness.
Toss in cooked penne, sliced chicken, and bacon. Loosen with pasta water as needed.
Taste and adjust salt and pepper. Serve hot with extra Parmesan.