Heat the olive oil in a 30 cm / 12-inch braising pan over medium heat. Add the onion, carrot, and celery, and cook for 6 to 8 minutes, stirring often, until softened and pale gold.
Add the garlic and cook for 1 minute until fragrant.
Season the palombo steaks with salt and pepper. Nestle them into the pan and sear for about 2 minutes per side, just enough to lightly color the surface.
Pour in the white wine and let it bubble for 1 to 2 minutes, scraping up any browned bits from the bottom of the pan.
Add the crushed tomatoes, bay leaf, chili flakes if using, and the fish stock. Bring to a gentle simmer.
Partially cover the pan and simmer for 12 to 15 minutes, spooning sauce over the fish occasionally, until it's nearly opaque throughout.
Stir in the peas and cook uncovered for 8 to 10 minutes more, until they're bright green and tender and the fish flakes easily at 145 F / 63 C.
Remove the bay leaf, stir in the parsley, and adjust the salt. Rest off heat for 2 minutes before serving.