Boil a large pot of water and plunge the octopus tentacles into it for 15 seconds. Repeat four more times, then let simmer for 30–40 minutes until tender but firm.

Line a terrine mold with plastic wrap. Cut the octopus tentacles to fit snugly inside. Toss with paprika and half the parsley, then layer in the mold. Cover tightly with plastic wrap and refrigerate overnight.

Boil the whole potatoes in salted water for 40–45 minutes until fork-tender. Peel and roughly mash them in a bowl. Add olive oil, sliced onion, paprika, remaining parsley, and sea salt. Mix well.

Unmold the terrine carefully and slice into thin, 5 mm pieces. Serve alongside the warm potato salad at room temperature.