Begin by rinsing the whole octopus thoroughly under cold running water, ensuring all dirt and impurities are removed. Pat it dry with a paper towel, paying extra attention to the suckers.

In a large pot, combine 8 cups of water, the halved yellow onion, smashed garlic cloves, bay leaves, black peppercorns, red wine vinegar, and salt. Bring this mixture to a rolling boil over medium-high heat.

If you're using optional aromatics like celery stalks and parsley stems, add them to the pot now. Reduce the heat to maintain a gentle simmer.

Carefully lower the cleaned octopus into the simmering broth using tongs, ensuring it’s fully submerged. Cover the pot loosely and let it simmer for 45 to 60 minutes.

Once cooked, turn off the heat and let the octopus rest in the broth for an additional 10 minutes.

After the resting time, carefully remove the octopus from the broth and place it on a cutting board. Slice the tentacles into portions, drizzle with olive oil and a squeeze of lemon juice.