Heat water to a boil for pasta, salt water and cook pasta to al dente.
Heat butter in a large sauce pot with butter, add garlic, cook 2 minutes then whisk in flour, cook 1 minute. Whisk in stock then cream, bring to a bubble and stir in Gorgonzola, about 2 minutes until melted. Stir in sage and a little pepper and cook 3 minutes more.
In a serving bowl toss hot pasta with sauce and fresh spinach to wilt in.