Prep the chicken. Pat the chicken dry and cut into bite-size pieces. Season with salt, pepper, and Italian seasoning.
Brown the chicken. Heat 1–2 tablespoons olive oil in a large, deep skillet or Dutch oven over medium-high. Add the chicken in a single layer.
Cook, stirring once or twice, until lightly browned and just cooked through, about 5–6 minutes. Transfer to a plate and cover to keep warm.
Sauté the aromatics. Lower heat to medium. Add a splash more oil if needed.
Add diced onion and a pinch of salt. Cook until softened, 3–4 minutes. Stir in garlic and red pepper flakes; cook 30 seconds until fragrant.
Deglaze and add liquids. Pour in the chicken broth, scraping up any brown bits from the bottom of the pan.
Bring to a simmer, then stir in the cream.
Cook the tortellini. Add the tortellini to the simmering liquid. Stir to prevent sticking. Cook according to package directions, usually 3–5 minutes, until just tender. Keep the heat to a gentle simmer so the cream doesn’t separate.
Return the chicken. Add the browned chicken and any juices back to the pot.
Simmer 1–2 minutes to warm through.
Finish the sauce. Stir in the Parmesan until melted and slightly thickened. Fold in the spinach and cook just until wilted, 1–2 minutes. Squeeze in lemon juice and taste. Adjust salt and pepper as needed.
Serve. Ladle into bowls and top with extra Parmesan and fresh herbs.
Serve hot.