In a large skillet, heat 4 tablespoons butter.  Cook leeks until soft (about 10 minutes).  Add vermouth and boil over high heat until liquid is reduced to 3 tablespoons (about 5 minutes).  Add cream and boil until thickened (about 3 minutes).  Sir in mustard, parsley and tarragon.  Taste for salt and pepper and keep warm.

Preheat oven to 400 degrees.

Sprinkle salmon with salt and pepper.  Dust calmon with flour.  Heat remaining butter in skillet and cook salmon about 3 minutes.  Turn salmon over and place skillet in 400 degree oven for another 5 minutes.  Transfer salmon to plates and spoon sauce over.