Dice garlic very finely, wash arugula and basil, dry and cut into strips, leaving small leaves whole. Heat the butter and olive oil in a non-stick frying pan, add the garlic and sauté briefly.
Dust with the flour and sweat until light. Deglaze with the chicken stock and half the milk, stir well and bring to the boil. Gradually add as much milk as needed to achieve the desired creaminess of the sauce. Simmer over low heat for about 10 minutes, stirring frequently, until the flour taste is gone. Now season to taste with salt, pepper, mustard and nutmeg, adding some more grained broth if necessary. Cook the pasta al dente.
Add ½ ladle of pasta water to the sauce. Drain the pasta. Bring the sauce to the boil briefly, remove from the heat and stir in the arugula and basil, tossing just briefly and serve immediately with the pasta.
Sprinkle with Parmesan cheese.