01 - Bring a large pot of generously salted water to a rolling boil. Add the cheese tortellini and cook according to package directions until al dente. Drain well and set aside.
02 - While the tortellini cooks, pat the diced chicken breasts dry and season evenly with salt, pepper, and Italian seasoning. Heat olive oil in a large skillet over medium-high heat. Add the chicken in a single layer and cook without disturbing for 2–3 minutes until golden on the first side. Continue cooking, stirring occasionally, until the chicken is deeply golden and cooked through, 5–7 minutes total. Transfer to a plate and tent loosely with foil.
03 - Reduce the skillet heat to medium. Melt the butter, then add the chopped onion. Sauté for 2–3 minutes until softened and translucent. Stir in the minced garlic and cook for 1 minute until fragrant, taking care not to let it brown.
04 - Pour the chicken broth into the skillet and deglaze, using a wooden spoon to scrape up all the caramelized browned bits from the bottom. Lower the heat to medium-low and stir in the heavy cream. Add the Parmesan cheese and stir continuously until the cheese has fully melted and the sauce begins to thicken slightly, about 3 minutes.
05 - Add the fresh baby spinach to the sauce and stir gently until just wilted, about 1–2 minutes.
06 - Return the seared chicken to the skillet along with the drained tortellini. Gently toss everything together until evenly coated in the creamy garlic sauce. Taste and adjust seasoning with additional salt and pepper as needed. Serve immediately, garnished with extra Parmesan and fresh chopped parsley if desired.