Bring a large pot of salted water to a rolling boil. Add the pasta and cook according to package directions until al dente. Before draining, reserve 1 cup of the pasta cooking water. Drain the pasta and set aside.

While the pasta cooks, heat a dry large skillet over medium heat. Add the chopped walnuts and toast for 3-5 minutes, stirring frequently, until fragrant and lightly golden. Be careful not to burn them. Remove from skillet and set aside.

In the same large skillet (no need to clean if you toasted walnuts), melt the butter over medium heat. Add the minced garlic and sauté for 1 minute until fragrant, being careful not to brown it.

Pour in the heavy cream. Bring to a gentle simmer, then reduce the heat to low. Add the crumbled Gorgonzola cheese and grated Parmesan. Whisk continuously until the cheeses are melted and the sauce is smooth.

Stir in the black pepper and red pepper flakes (if using). If the sauce seems too thick, add a tablespoon or two of the reserved pasta water until it. reaches your desired consistency. Taste and adjust seasoning as needed; blue cheese is salty, so you might not need additional salt.

Add the cooked, drained pasta to the skillet with the sauce. Toss well to coat all the pasta evenly.

Add the baby spinach and toasted walnuts to the pasta. Continue to toss gently until the spinach wilts, which should only take 1-2 minutes.

Divide the pasta among serving bowls. Garnish with fresh chopped parsley and an extra sprinkle of grated Parmesan cheese, if desired. Serve immediately.