Cook the tortellini according to the packet's instructions. Dried tortellini will need a longer cooking time, depending on the brand between 12 and 15 minutes. Fresh tortellini will only need 2-3 minutes of cooking time. Drain very well.

Preheat the oven to 200 degrees Celsius/ 400 degrees Fahrenheit.

While the water for the pasta and the tortellini are cooking, halve and deseed the tomatoes. Chop them into small cubes.

Chop the onion and the garlic as well.

Heat the oil in a pan and saute the onion for about 3-4 minutes. Add the garlic and stir for about 1 minute. Add the tomato paste and stir well.

Pour in the vegetable broth and add the chopped tomatoes to the pan as well.
Add some salt and pepper and cook the mixture on low heat for about 15 minutes. Add a little more water or vegetable broth if the sauce becomes too dry.

Take off the heat, add the chopped green olives, basil, and chili flakes. Adjust the taste with salt and pepper.

Mix the tomato sauce with the cooked tortellini. Add the chopped mozzarella and stir well.

Place the tortellini mixture into a casserole dish of about 19x23 cm/ 7.5-9 inches.

Bake for about 15 minutes or until the tortellini casserole is heated through and nicely colored on top.