Prepare the Fish: Mix the batter ingredients in a bowl until smooth and set aside for at least 10 minutes. Cut the fish fillets into square-ish shapes slightly larger than the buns, as they will shrink when cooked.
Toast the Breadcrumbs: Preheat the oven to 180°C (350°F). Spread the breadcrumbs on a baking tray and spray them with oil directly from above (avoid spraying from the side). Bake for 3 to 5 minutes, or until golden brown and slightly crispy. Once done, immediately transfer the breadcrumbs to a bowl.
Prepare the Baking Tray: Brush the baking tray clean and spray it lightly with oil again.
Coat the Fish: Carefully coat each fish fillet in the batter (handle gently as the fillets can be delicate). Dredge the battered fish in the toasted breadcrumbs, using your other hand to cover the top and sides, pressing the breadcrumbs to adhere. Transfer the coated fish to the prepared tray and repeat with the remaining fillets.
Bake the Fish: Bake the fish in the preheated oven for 12 to 15 minutes or until cooked through. Once cooked, immediately top each piece with a slice of cheese, allowing the residual heat to melt it slightly (avoid placing back in the oven to melt processed cheese, as it can become plasticky).
Assemble the Sandwiches: Cut the burger buns in half and microwave them for about 20 seconds to warm them through (this keeps them soft). Top the bottom half of each bun with a piece of the fish and cheese, then dollop with tartare sauce.
Serve: Place the top half of the bun on top and serve immediately. Enjoy your delicious crispy fish sandwiches!