Fill a large pot with water and a generous palmful of salt. Bring to a rolling boil. Add the pasta and cook until just shy of al dente. Reserve about one cup of pasta water before draining.

Melt the butter in a large skillet over medium heat. Add the finely diced onion and cook until translucent. Stir in the minced garlic and sauté until fragrant.

Turn the heat down to low. Pour in the heavy cream and let it come to a gentle simmer. Add the crumbled blue cheese and stir until melted.

Add the grated Parmesan and a splash of the reserved pasta water. Whisk gently until the sauce is smooth and thick enough to coat the back of a spoon. If too thick, add a bit more pasta water.

Toss the cooked pasta into the skillet. Use tongs or a large spoon to ensure every strand is fully enrobed in the sauce. Stir in the toasted walnuts and the fresh lemon zest.

Taste and adjust for seasoning. Serve immediately while the sauce is at its glossiest.