In a large pot over high heat, add wine and mussels then cover pot with lid and steam until mussels just start to open (discard any that don’t open). Remove from heat and set mussels aside to cool.
When cool enough to handle, remove tongues and beards (see below) then separate mussel meat from shells.
In a blender, add mussel meat and pulse to a chunky paste (make sure it’s not puréed as you still want to see some mussel texture inside). Set aside.
In a large pot over medium heat, melt butter then add flour and cook, stirring continuously, for 4 minutes. Add milk slowly, stirring continuously to avoid lumps. Bring to the boil then reduce to a simmer and cook for 15 minutes, being careful that it doesn’t catch on the bottom of the pot. Add stock concentrate, water and Indian seasoning. Bring to the boil then set aside (this is a velouté).
In a separate pan, heat oil over medium heat. Add onion and garlic and cook, stirring regularly, until very soft and translucent. Add to velouté along with blended mussels. Place pot over medium heat and bring to the boil (again, being careful that it doesn’t catch).
Add Tio Pepe and nutmeg, season with salt and pepper to taste, and serve.