Preheat oven to 400 degrees F (200 degrees C).

Place swai fish in a glass casserole dish and season with salt and black pepper.

Heat olive oil in a large skillet over medium heat; cook and stir onion until softened, about 10 minutes. Add garlic and cook until fragrant, about 1 minute. Pour tomatoes over onion mixture; cook and stir until heated through, about 5 minutes. Spoon tomato mixture over fish.

Bake in the preheated oven until fish flakes easily with a fork, about 20 minutes.