Pat the cuttlefish strips dry with paper towels. Moisture left on the surface stops it from browning.
Heat olive oil in a wide braising pan over medium heat. Add onion and garlic, cook 4 to 5 minutes until soft and translucent.
Raise heat to medium-high, add cuttlefish, cook 3 to 4 minutes, stirring, until it releases liquid and turns opaque.
Pour in white wine, let it bubble and reduce by half, about 2 minutes.
Stir in tomato passata, salt, pepper, and chili flakes if using. Lower heat, cover partially, and simmer 35 to 40 minutes, stirring occasionally, until cuttlefish is fork-tender. Add a splash of water if the sauce reduces too fast.
Add peas, cover, and simmer 8 to 10 minutes until peas are tender and bright green.
Stir in parsley, taste, and adjust salt. Serve hot.