1Pat fish dry and sprinkle both sides with salt and pepper. Set aside.
2Combine tomatoes, celery, onion, olives, capers, oil, garlic and crushed red pepper in a large skillet and toss to mix well. Layer potato slices over the vegetables to cover them completely.
3Cover the skillet and place over medium-low heat. Cook, adjusting the heat to keep a steady simmer and shaking the pan from time to time—but do not stir the vegetables—until the potatoes are starting to soften, about 20 minutes.
4Place the fish on top of the potatoes, cover and continue cooking until the fish is opaque in the middle, 10 to 12 minutes more. Serve sprinkled with parsley and more capers, if desired.