If using frozen swai, remove them from the packaging and allow them to thaw overnight in the refrigerator or at room temperature until completely thawed. If thawing at room temperature, do not let the fish sit longer than four hours at room temperature. Heat a large nonstick skillet over medium high heat until the pan is very hot. Add 1 tbsp of oil to the pan and swirl to coat. Add the first two fillets cooking for as long as necessarily to achieve a nicely browned bottom. Add ¼ tsp of kosher salt for two fillets and divide ½ tsp black pepper between the 6 fillets. Flip the fish and repeat on the other side. This fish is hard to overcook and is actually nicer with quite a few crispy spots so do not be afraid to give it a few extra minutes. If the heat on the fish seems too hot, you can turn the heat down slightly as the fish is crisping. Once fish is cooked, remove to a plate and repeat with the remaining fillets.
While the fish is cooking, slice the cucumber on a bias into very thin slices. Shingle them on top of each other and cut them the other direction into a thin julienne. Cut the ends of the red onion and peel it. Slice ¼ of the red onion into a very thin julienne. Add the cucumber and red onion to a medium sized bowl. Add washed and chopped cilantro to this mixture as well. Do not top with salt, pepper, and lime juice until all the fillets are cooked and you are ready to serve.
Finish up cooking the last couple fillets and plate them together.
Toss the salad with juice from ½ a lime, ¼ tsp salt and ¼ tsp black pepper. Place on top of the hot fish fillets and serve immediately.