Bring a large pot of lightly salted water to a boil. Boil the tagliatelle until tender yet firm to the bite, 8 to 10 minutes.
Meanwhile, heat cream in a saucepan over low heat until warm; stir in blue cheese, lemon juice, garlic powder, nutmeg, oregano, basil, salt, white pepper, and black pepper. Cook very gently until blue cheese has melted and sauce is hot; make sure the sauce doesn't boil.
Drain tagliatelle and return to the pot. Drizzle olive oil over top and toss gently to coat. Transfer to a serving dish.
Pour blue cheese sauce over tagliatelle. Top with walnuts to serve.