Bring a large pot of salted water to a rolling boil and add the pasta. Cook for 8–10 minutes, stirring occasionally, until the pasta is just tender with a slight bite to it.
While the pasta is cooking, melt the butter in a large pan over medium heat. Add the garlic and cook, stirring constantly, for about 1 minute until it becomes fragrant but doesn’t brown. Toss in the mushrooms and continue cooking, stirring occasionally, for 2 minutes until they start to soften and release their juices. Pour in the cream and increase the heat to bring it to a boil. Let it bubble for 2–3 minutes, stirring from time to time, until the sauce reduces slightly and reaches a thick, coating consistency.
Once the pasta is cooked, drain it well in a colander and immediately add it to the pan with the mushroom and cream sauce. If using the egg, add it now and stir everything together vigorously to combine, allowing the residual heat to warm the mixture through. Fold in the spinach, crumbled Stilton cheese, a squeeze of lemon juice, a pinch of nutmeg, and pepper to taste. Toss everything together thoroughly until the pasta is evenly coated with the creamy, cheese-laden sauce and the spinach has wilted into the dish. Transfer to warmed plates and serve immediately while hot and luscious.
