In a large heavy bottom pot, begin heating the oil over medium high heat until it reaches 350 degrees F.
While the oil is heating, in a large mixing bowl, whisk together the flour, garlic powder, black pepper, and salt. Add the egg and beer and whisk until just combined. Don't over mix!
Dry off the cod fillets with a paper towel. Dip the fillets in the beer batter mixture and coat completely. Drop into fully heated oil and fry until golden brown on all sides, about 5 minutes. Make sure to keep the oil at 350 degrees F while frying. Remove to a paper towel-lined plate.
In a small mixing bowl, mix together the sriracha and tartar sauce. Remove 2 tbsp to a separate bowl. Toss the cole slaw with remaining mixture until fully coated. Set aside.
Open the hoagie rolls and lightly butter the inside. Arrange open-faced on a cookie sheet and broil in the oven on high until just golden brown, about 1 minute.
Spread tartar sauce sriracha mixture on the inside of the hoagie rolls. Arrange one cod fillet on each of the hoagie rolls. Top with prepared cole slaw.