If using dried tortellini, cook as per package instructions. Keep aside.
In a pot, heat oil. Add chopped onion and garlic. Saute for a minute until the onions have softened.
Add flour and saute for about a minute.
Add diced tomatoes, mix well and cook for 2-3 minutes.
Stir in tomato passata, chicken stock, salt, pepper, and sugar. Bring it to a boil.
Reduce heat and stir in heavy cream. Add cooked tortellini and roughly chopped Italian basil. Simmer for 5 minutes and switch off the heat. (if using fresh/ refrigerated tortellini cook it for longer until the pasta is cooked through).
Serve warm with freshly grated parmesan cheese on top and garnish with fresh basil.