Heat olive oil in a large heavy pot over medium heat until shimmering. Add the sliced onions and cook, stirring occasionally, until they soften and begin to caramelize, about 10-15 minutes. The onions should turn a rich golden-brown and release a sweet aroma.
Add the minced garlic, ground cinnamon, and paprika to the onions. Stir for about 1 minute until fragrant, making sure not to burn the garlic. The mixture should smell warm and aromatic.
Pour in the red wine vinegar, scraping the bottom of the pot to deglaze and lifting up any flavorful brown bits. Cook for 2-3 minutes until the vinegar evaporates slightly, blending with the onions and spices.
Place the octopus into the pot, breaking it into manageable pieces if needed. Pour over the broth, making sure the octopus is mostly submerged. Bring the mixture to a gentle simmer.
Reduce the heat to low, cover the pot, and let everything simmer gently for about 1.5 to 2 hours. The octopus should become tender, and the sauce will thicken and deepen in flavor as it cooks slowly.
Uncover the pot in the last 15 minutes to allow the sauce to reduce slightly, achieving a rich, glossy appearance. Season with salt and pepper to taste. The octopus will be tender, with a slightly glazed surface.
Gently stir the stew, checking the seasoning and adjusting salt and pepper if needed. Serve hot, ideally with crusty bread or alongside a fresh Greek salad.