Add the rosemary, peppercorns, and milk to a saucepan over a medium setting.
Bring to a simmer for about 5 minutes.
Remove from the heat, strain out the rosemary and peppercorns, and allow the milk to cool.
Once cooled, melt the butter in a saucepan, and add the flour. Stir together, and allow it to cook over a medium setting for a 3 to 5 minutes without burning.
Dribble in the seasoned milk, stirring constantly. Heat, continuing to stir, until the sauce is thick.
Remove from the heat and add the remaining ingredients, stirring to incorporate them.
Serve over the fish immediately.