Pat the monkfish chunks dry with paper towel and season lightly with salt. Set aside.
Heat the olive oil in a 30 cm / 12-inch braising pan over medium heat. Add the onion and cook for 5 to 6 minutes until soft and translucent.
Add the garlic and chili flakes, if using. Cook for 1 minute until fragrant, stirring so it doesn't catch.
Pour in the wine and simmer for 2 minutes, scraping up any browned bits from the bottom of the pan.
Add the crushed tomatoes, olives, and capers. Season with salt and pepper, then simmer uncovered for 12 minutes until the sauce thickens slightly and coats the back of a spoon.
Nestle the monkfish chunks into the sauce in a single layer. Cover and cook for 8 to 10 minutes, turning the pieces once.
Check doneness with an instant-read thermometer at the thickest chunk. It's ready at 145 F / 63 C, when the flesh is opaque and flakes under light pressure.
Stir in the parsley and taste, adjusting salt if needed. Serve straight from the pan with crusty bread or polenta.