mustard and soy sauce recipe?

Every time I go to a different sushi place and get the “Cajun” fish it’s topped with this sauce. I asked today and they said it’s mustard soy sauce but when I look that up no recipe looks similar. Does anyone know a good recipe with measurements and is it really just mustard and soy?

Edit: pic of the sauce in question https://imgur.com/a/qwuWbxE