From Instant Pot Italian: "There’s a different version of this delicious fish and fennel soup, called buridda, in every harbor of northwestern Italy. Local red mullet or bass is traditional, but I opt for extra-firm fish like sustainable swordfish, halibut, or thick cod fillets, as they hold their shape better in the Instant Pot." —Food52
Put the oil in the pot, select SAUTÉ, and adjust to NORMAL/MEDIUM heat. When the oil is hot, add the onion, fennel, bell pepper, and oregano and cook, stirring occasionally, until tender, 5 minutes. Add the garlic and tomato paste and cook, stirring frequently, until the garlic is fragrant, 45 seconds. Add the vermouth and simmer to burn off some of the alcohol, 1 minute. Press CANCEL.
Add the broth, potatoes, tomatoes, bay leaf, ½ teaspoon salt, and several grinds of black pepper. Place the fish on top of the top of the vegetables, but do not submerge it. Lock on the lid, select the PRESSURE COOK function, and adjust to HIGH pressure for 5 minutes. Make sure the steam valve is in the “Sealing” position and that the “Keep Warm” button is off.
When the cooking time is up, quick-release the pressure. Use a slotted spoon to transfer the fish to a cutting board. If using swordfish, remove the skin, if present, and discard. Cut the fish into bite-size pieces and set aside.
Add the clams and/or mussels to the pot and stir very gently to submerge them in the cooking liquid. Select SAUTÉ, adjust to LESS/LOW heat, and cook, uncovered, until the shells open, 5 to 8 minutes. Discard any unopened shellfish. Season the soup with salt and black pepper and discard the bay leaf. Return the fish to the pot and stir gently to combine. Press CANCEL.
Serve the stew with the bread and lemon wedges on the side.