Preheat oven to 375°F (190°C). Grease a 9x13-inch baking dish.
Place the first measure of butter and oil into the pan and sear the chicken for a couple of minutes. It doesn't need to be cooked through as it will cook during the baking time.
Add the cream, stock, salt, pepper, and caramelized onions. Cook for 8 minutes
At the end of the cooking time, add the spinach and Parmesan cheese
Combine the second measure of butter and flour to form a paste. Add to the sauce to thicken slightly. You still want the mixture to be on the thinner side. So don't be tempted to thicken it more. It will continue to thicken as the pasta cooks.
Assemble the bake – Pour the mixture into the prepared baking dish. Add the two uncooked packets of tortellini and thoroughly mix together. Cover the baking dish with foil.
Bake for 10 minutes. Then, after 10 minutes, stir it and cook for another 5 minutes. Take the foil off and sprinkle the grated cheese evenly over the top, and bake for a further 5 minutes or until the cheese is melted and bubbly with a golden top.
Garnish & Serve – Sprinkle with fresh parsley and serve hot!