NSFL

In france you have to go to foreign restantrants to get repetitive menus (pizzerias, kebab places, etc)

I’d say some gratins (cauliflower, broccoli, etc) or croque-monsieur would be good candidates for that sauce and the nutmeg really makes the sauce pop (taste it for spiciness btw and adjust).

Regardless though, it is indeed a mother sauce as others have said

Pretty banger IMO. Do you include salt, pepper and nutmeg when you make it ?