Preheat oven to 375ºF, or whichever temperature it suggests on the package of the biscuits you're using. Line a baking sheet with parchment paper and set aside.
Drain both cans of mackerel well and dump into a large bowl. Break the fillets into chunks with a fork, almost resembling pulled chicken. Add the hot sauce along with a pinch of salt, and toss to coat. You can also use celery salt instead of kosher salt here.
Create a lightly floured surface. Open the biscuit and, one biscuit at a time, roll each one out into a flattened circle. Smear the center of each biscuit with a little hot sauce leaving a 1/2-inch border around the edges of the biscuit.
Place about 2 tsp worth of mackerel in the center of each biscuits, then topping with 1 TBSP of mozzarella cheese and a crack of black pepper. Pull the edges of each biscuit up and over the filling, pinching the dough together at the seams and creating somewhat of a sealed pouch. Place the biscuits onto the lined baking sheet, seam side down.
Brush a bit of olive oil over each biscuit; sprinkle with a bit of celery salt. You could also use garlic salt, and/or add some dried herbs if you wish.
Bake in the oven for 20 minutes or until golden brown. (Refer to your biscuit's packaging for cooking instructions and take those into consideration.)
When done, let them cool for five minutes, then sprinkle with fresh parsley (optional) and serve with blue cheese dressing and extra hot sauce for dipping.