Pat the fish dry with paper towels and cut it into 5 cm / 2-inch pieces. Keep it in the fridge while you make the sauce. Slice the onions thin and slice the garlic.
Heat the olive oil in a 30 cm / 12-inch deep sauté pan over medium-low heat. Add the onions and 1 tsp salt. Cook for 15 minutes, stirring every few minutes, until the onions are soft and light golden.
Add the garlic and cook for 1 minute. Stir in the sweet paprika, hot paprika, black pepper, and tomato paste, and cook for 1 minute, stirring the whole time. Paprika burns fast and turns bitter, so keep the heat at medium-low and move on right away.
Pour in the crushed tomatoes and water, and stir well. Bring to a gentle simmer, partly cover the pan, and cook for 15 minutes, until the sauce is thick enough to coat the back of a spoon.
Place the fish in the sauce in a single layer and spoon some sauce over the top. Cover and cook at a gentle simmer for 8 minutes, until the fish is opaque all the way through, flakes easily with a fork, and reads 145 F / 63 C in the thickest piece. Don't stir. Shake the pan gently instead.
Take the pan off the heat and let it sit covered for 5 minutes. Taste the sauce and add a pinch of salt if it needs it. Sprinkle with parsley and serve with lemon wedges.