Prepare Tomato Sauce: In a large skillet, heat half of the olive oil over medium heat, adding finely sliced onion. Sauté for about 15 minutes, stirring occasionally, until the onion is soft and translucent but not colored. Next, add the chopped ripe tomatoes, season with sea salt, and bring to a gentle simmer. Let the sauce cook for 45 minutes, stirring occasionally, until it thickens and develops rich flavors.

Flavor the Sauce: While the tomato sauce simmers, heat the remaining olive oil in a small pan over low heat. Add minced garlic and cook until golden, which should take approximately 2 minutes. Mix in the fresh basil leaves, then strain the oil into the tomato sauce. Stir well to combine, adjusting the sauce's consistency with water if needed, and taste for additional seasoning.

Preheat Oven: Preheat your oven to 200°C (gas mark 6). This temperature ensures that the stuffed calamari will cook evenly and become tender.

Prepare Filling: Bring a pot of water to a boil and blanch the frozen peas for about 30 seconds. Drain and set aside to cool. In a food processor, combine squid, egg whites, chopped romano peppers, and chives, blending until you achieve a coarse filling mixture.

Stuff Squid: Carefully fill the squid with the pea and vegetable mixture, leaving a small gap at the top to allow for expansion during cooking. Secure the openings with the squid's tentacle or a toothpick.

Cook Calamari: In the same pan used for the garlic, add a splash of extra olive oil and gently fry the stuffed calamari for about 3-4 minutes until golden brown. Pour in white wine to deglaze the pan. Finally, add the prepared tomato sauce and fish stock and stir gently before transferring to the preheated oven to bake for 20 minutes.

Serve and Enjoy: After baking, the stuffed calamari should be perfectly tender and infused with the tomato sauce's flavors. Plate the calamari elegantly, drizzling with additional sauce.