Preheat oven to 190℃ (375°F). Grease a 9x13-inch baking dish.
Place the first measure of butter and oil into the pan and sear the chicken for a few minutes. The chicken does not need to be cooked through as it will continue to cook in the oven.
Add the cream, stock, salt, pepper, and caramelized onions. Cook for 8 minutes.
Stir through 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.
To assemble the bake – pour the mixture into the prepared baking dish. Add the two uncooked packets of tortellini and stir to combine. 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!