Preheat the oven to 180C / 360F.

Heat a little olive oil in a shallow casserole dish (one that can go between the stove and oven). Saute your onion and garlic until soft.

Prepare the spinach. Microwave it until defrosted, then squeeze out as much of the liquid as you can. I find the best way to do this is to put the spinach in a sieve and use a bowl to press down on it which squeezes the water out. Once finished, add it to the pan and mix it through the onion and garlic mixture for a minute or two.

Add the cream cheese and the parmesan style cheese (not the mozzarella - this is for topping!) and mix through over very low heat until it's all combined and the cheese is melted.

Add the vegetable stock and tortellini to the pan. The sauce will be very runny, but don't worry, the tortellini will drink it up while it cooks! Cover, then place in the oven for 20 minutes.

Remove from the oven and uncover. The tortellini should have plumped out and the sauce will be much thicker, though still fairly liquid (don't worry, it will continue to thicken). Stir it through.

Put the oven's heat up to 200C / 400F. Add the mozzarella cheese over the top and then place back in the oven for a further 5-10 minutes until the cheese is bubbling and browned.

Remove from the oven and let it sit for out for 5-10 minutes before serving. This time will allow the sauce to settle.