How to Make It
Bring a large saucepan of salted water to a boil with the onion and bay leaves. Using tongs, carefully dip the octopus into the boiling water 3 times, then leave it in the water. Cook the octopus over moderately low heat until tender, about 1 hour. Remove from the heat and let the octopus stand in the water for 10 minutes; drain. Cut the octopus into 1/2-inch pieces.
In a medium saucepan, cover the potatoes with water and add salt. Bring to a boil and simmer over moderate heat until just tender, about 10 minutes. Drain and transfer to a bowl. Toss the potatoes with the 3 tablespoons of olive oil and the thyme; season with salt and pepper.
In a grill pan, cook the chorizo over moderately high heat until warmed through, 2 minutes. Transfer to a bowl. Add the potatoes and octopus to the pan and cook until hot and the potatoes are golden in spots, 5 minutes. Add to the chorizo, season with salt and pepper and toss. Drizzle with oil, garnish with shiso, if using, and serve.