Heat the butter in a large skillet or Dutch oven over medium-high heat. Cook the bacon until crisp and golden, about 10 minutes. Remove with a slotted spoon and set aside, leaving the drippings.
Lower the heat slightly and cook the onion until soft, about 3 minutes. Add the garlic and fennel seeds (if using) and cook for another 1-2 minutes.
Add the sausage meat, increasing the heat slightly if needed. Cook, breaking it up, until mostly cooked through, about 5-6 minutes.
Bring a pot of salted water to the boil and cook the pasta 1-2 minutes less than the package instructions.
Add the passata and cream to the sausage mixture and bring to a very gentle simmer.
Add 40 grams of the parmesan in and mix in until melted.
Add the bacon back in with the lemon juice and black pepper. Taste and adjust salt, it should be slightly on the salty side as the pasta will dilute the flavor.
Simmer gently on low heat for 10 minutes while the pasta cooks.
Reserve 1 cup of the sauce, then mix the drained pasta into the remaining sauce. Remove from heat and let cool slightly.