Start by placing your large skillet over medium-high heat and adding both the olive oil and butter. Once the butter has melted and is bubbling slightly, toss in your chopped onions. Cook them for about five minutes until they soften and start to become translucent, stirring occasionally to prevent burning.
Next, add the sliced cremini mushrooms, minced garlic, and Italian seasoning directly into the pan with the onions. Continue to sauté everything together for another five minutes. You want the mushrooms to release their moisture and develop a nice golden-brown color, which adds tremendous flavor to the final sauce.
Pour the chicken broth and heavy cream into the skillet, then gently stir in the uncooked tortellini. Cover the pan with a lid and reduce the heat to medium. Let it cook for about five minutes; the pasta will cook directly in the liquid, absorbing the flavors while the starch helps thicken the sauce naturally.
Remove the lid and give everything a good stir; the sauce should be thicker now. Add the fresh spinach and toss it gently for about two minutes until it wilts completely into the sauce. Finally, stir in the grated parmesan cheese until melted, season with salt and pepper to taste, and serve immediately.