Cook the octopus tentacles in a soup pot over low heat, covered with simmering salted fish stock or vegetable stock, along with a few sprigs of fresh thyme.
Continue cooking the octopus over low heat for roughly three or four hours, or until the octopus tentacles are completely tender.
Remove the tender octopus tentacles from the cooking liquid, and cut the meat into bite-sized pieces.
Reserve the braised octopus tentacles and the cooking liquid for later.
In a large sauté pan, add ¼ cup of olive oil and increase the heat to medium-high.
Add the diced white onion and leeks, and cook while stirring for ten minutes.
Add the minced garlic and the white wine, and cook until the white wine has mostly evaporated, or roughly 15 minutes.
Add the tomato puree, red pepper flakes, dried oregano, freshly-ground black pepper, and smoked pimentón.
Reduce the heat to medium-low, and continue to cook for an additional twenty minutes while stirring occasionally.
Add the Bomba rice, along with one third of the cooking liquid that you used to braise the octopus.
Add the fresh parsley, and cook over medium-low heat until the liquid has been absorbed by the rice.
Add more liquid one cup of seafood stock at a time, stirring slowly until the rice is cooked.
When the rice is fully cooked, add in the braised octopus from earlier, along with two Tablespoons of butter.
Serve the rice immediately, in warm bowls.