In a large pot, boil the potatoes until tender, about 15 minutes. Drain and mash them in a bowl.
In a food processor, combine the fish fillets, green onions, mayonnaise, Dijon mustard, garlic powder, salt, black pepper, paprika, and cayenne pepper. Pulse until the mixture is well combined but still slightly chunky.
In a large mixing bowl, combine the fish mixture with the mashed potatoes and breadcrumbs. Mix until fully combined.
Form the mixture into patties, about 3 inches in diameter and 1/2 inch thick. Place them on a baking sheet lined with parchment paper.
Refrigerate the patties for at least 30 minutes to help them firm up.
Heat about 1/4 inch of vegetable oil in a large skillet over medium heat.
Dredge each fish cake in flour, then dip in the beaten egg, allowing excess to drip off.
Carefully place the patties in the hot oil and fry for 3-4 minutes on each side, or until golden brown and crispy.
Remove the fish cakes from the skillet and place them on paper towels to drain excess oil.
Serve warm with sweet chili sauce for dipping.