Preheat oven to 375° F.
Spread out your crescent roll dough and cut out the crescent shapes; set them aside.
Take one of the crescent cut outs and spread 1/2 tsp of stone ground mustard into the center of the widened end. Then, take about a TBSP of cheese and place it on top of the mustard. Top that off with about a TBSP of King Oscar mackerel. Top with a little extra cheese if you wish, then roll the pastry upward toward the pointed end, tucking the ingredients as you go. Curl the sides. Voila! A nicely stuffed crescent roll. Place it on the sheet pan. Repeat this until you've stuffed all of the crescent rolls. Crescent roll dough is pretty sturdy, so don't be afraid to stuff generously.
Once all of your crescents are rolled, place the sheet pan in the oven and bake for 10-14 minutes, or until the crescent roll dough is golden in color.
Transfer the remainder of the stone ground mustard to a small dish for dipping.
Serve warm.