Season and sear the chicken: Pat the chicken dry and season with salt, pepper, and half the Italian seasoning. Heat olive oil in a large skillet over medium-high.
Add the chicken in a single layer and cook until golden and just cooked through, 5–7 minutes. Transfer to a plate.
Build the flavor base: Reduce heat to medium. Add butter to the skillet.
When melted, stir in garlic, remaining Italian seasoning, and red pepper flakes. Cook 30–60 seconds, just until fragrant. Don’t let the garlic brown.
Make the sauce: Pour in the chicken broth, scraping up browned bits.
Simmer 2 minutes to reduce slightly. Stir in the cream and bring to a gentle simmer.
Add the tortellini: Stir in the tortellini. Cover and cook over medium-low, stirring once or twice, until the tortellini are tender and the sauce has thickened slightly, about 4–6 minutes (check package directions).
Finish with cheese and spinach: Stir in Parmesan until melted and smooth.
Add the spinach and cook until just wilted, 1–2 minutes. If the sauce feels too thick, splash in a bit more broth or cream.
Return the chicken: Add the cooked chicken and any juices back to the skillet. Toss to coat and warm through, 1 minute.
Taste and adjust seasoning with salt and pepper. Add lemon juice if using for brightness.
Serve: Garnish with parsley and extra Parmesan. Serve hot straight from the skillet.