Remove your sausage from its casing, and cube it, sear it in a pan until it starts to release some of its fat and break in into smaller pieces, then drain it.

To the same pan add your onion and garlic, and allow it to sauté and soften. Add back the sausage and your passata, bay leaf, oregano and cook for at least one hour. Then add your fresh basil and allow it to continue cooking until the oil separates. I prefer to add the basil later, as I feel if it cooks too long it loses flavor.