Clean the octopus by rinsing thoroughly under cold water and removing the beak if necessary. Pat dry with paper towels.

In a large pot, combine the cleaned octopus with sliced onion, halved lemons, garlic cloves, bay leaves, peppercorns, and a pinch of salt. Cover with water and simmer for 45-60 minutes until fork-tender.

Allow the octopus to cool in the cooking liquid for 30 minutes to retain moisture.

Prepare the vinaigrette by whisking together olive oil, vinegar or lemon juice, chopped parsley, oregano, salt, black pepper, red pepper flakes, and lemon zest.

Cut the cooled octopus into pieces and toss with half of the vinaigrette. Refrigerate for 30 minutes or up to 24 hours.

Preheat the grill to medium-high heat (450°F) and brush the octopus pieces with oil.

Grill the marinated octopus for 2-4 minutes per side until charred.

Let the octopus rest, slice if needed, drizzle with remaining vinaigrette, and serve with lemon wedges.