In a large sauté pan on medium-high heat pour in enough vegetable oil to fill it up to ¾ inch. Heat the oil to 350°F. Arrange a cooling rack on top of a paper towel-lined sheet tray.
Using a fork, lightly pierce the tortillas a few times. Fry the tortillas in two batches, taking care not to overcrowd the pan, for 15 to 20 seconds on each side or until they’re a light golden brown. Use a spider or slotted spoon to transfer to the cooling rack. Season with kosher salt.
Heat a grill or grill pan on medium-high heat. Season the fish with salt and pepper and both sides before drizzling a little oil on both sides. Alternatively, you can grease the grill or grill pan after it is heated. Cook for 3 to 4 minutes until there are golden brown grill marks before flipping; cook for an additional 3 to 4 minutes, depending on the thickness of the filets.
Use a fork to flake the fish before dividing it amongst the 4 tostadas. Top with the slaw and salsa. Serve immediately.