Soften the onion and pepper in the oil over medium heat, 8 to 10 minutes, until the pepper genuinely slumps. Crisp peppers read raw no matter how long the sauce simmers afterward.
Add the garlic and smoked paprika, stirring 30 seconds until the spice smells toasted rather than dusty.
Pour in the crushed tomatoes and salt, and simmer uncovered 10 minutes until the sauce thickens enough to hold a brief trail behind a spoon.
Nestle the cod fillets into the sauce, spooning some over each one so they are half submerged.
Cover, drop the heat to low, and poach 8 to 10 minutes, until the fish flakes at the thickest point under gentle pressure.
Kill the heat and let it stand 2 minutes before serving straight from the pan.