Add it gradually while tossing. The sauce is ready when it looks silky and lightly coats the pasta rather than pooling at the bottom of the pan.
Two cheeses, two jobs
Parmigiano Reggiano and Pecorino Romano may look interchangeable, but they behave differently.
Parmigiano contributes nuttiness, sweetness and depth. Pecorino brings a sharper, saltier edge. Parmigiano alone can seem too gentle beside the sausage, while too much Pecorino can overwhelm the fennel.
Together, they provide the balance the dish needs. Add them away from direct heat so they melt smoothly rather than clumping, and taste before adding any additional salt.
What the lemon is doing
The lemon zest shouldn’t make the pasta taste lemony.
Its purpose is to lift the heavier flavors—the sausage, mushrooms, butter and cheese—and make the fennel seem fresher.
You shouldn’t necessarily identify lemon as a separate ingredient. You should simply notice that everything tastes a little more alive.
That’s often the difference between richness and heaviness.