Add the oil and butter to a deep skillet over medium-high heat. Sauté the onion for 5 minutes.
Meanwhile, prep your mushrooms (I like to prep as I go to save time).
Add the mushrooms, garlic, and Italian seasoning to the pan. Continue sautéing for another 5 minutes.
Add the chicken broth, cream, and tortellini to the skillet. Give it a good stir and then cover the pan. Reduce the heat to medium and cook for 5 minutes.
Uncover the pan and stir it. The tortellini will be cooked and the sauce should have reduced/thickened up a bit. Add in the spinach and stir continuously until it's wilted (about a minute or two). I found the sauce plenty as-is, but feel free to add a splash more cream if needed.
Stir in the parmesan and season with salt & pepper as needed. Serve immediately.