If using frozen squid tubes, thaw them completely. Clean if necessary and pat dry with paper towels.
In a bowl, combine 5 tablespoons of olive oil, the juice from the lemon, chilli flakes, garlic granules, and sea salt. Stir to make the marinade.
Using a sharp knife, score the squid tubes by making shallow diagonal cuts across the surface. Place in the marinade, cover, and refrigerate for at least 2 hours.
Bring a pot of salted water to a boil over high heat. Add some of the marinade to the water. Place the squid tubes in the boiling water and cook for 2-3 minutes. Remove from heat and let steep for 10 minutes.
While the squid is steeping, dice the garlic and finely chop the parsley. In a large pan over medium heat, warm the remaining marinade and remaining olive oil. Add the diced garlic and chopped parsley, stirring occasionally until fragrant, about 1-2 minutes.
Remove the squid tubes from the cooking water and cut into bite-sized pieces. Add to the pan, increase heat to medium-high, and bring to a boil. If the mixture seems dry, add a splash of the cooking water. Season with fleur de sel and serve.