To prepare octopus for cooking, remove the mouth section, rinse well. Add the octopus to a large pot of salted water over high heat. Bring to a boil, then reduce the heat and simmer for 30 to 35 minutes. Once the octopus is light pink and soft to the touch, remove from heat, drain and cool. Once cool, rinse and remove outer skin of tentacles, cut the head portion open, remove the insides and rinse again. Cut the octopus into 1/2 by 1/2-inch pieces.
In a large bowl, add the cucumbers, onions, garlic, thyme, sambal, sea salt, white pepper, olive oil, white balsamic vinegar, and lemon juice. Toss to combine and well mixed. Add the octopus, tossing to coat with the dressing, then add the tomatoes and chives. Gently toss, being careful not to break or bruise the tomatoes and chives. Cover and refrigerate for 1 to 2 hours. Transfer the salad to serving plates with a slotted spoon and drizzle with remaining dressing.