Make the broth. Heat the oil in a medium soup pot or Dutch oven over medium heat and add the garlic. Cook, stirring, until golden, 2 to 3 minutes, and add the onions, leeks, celery, and parsley. Turn the heat to medium-high and cook, stirring, for 2 minutes, or until the garlic begins to brown. Stir in the tomatoes and shrimp or fish, then add the water and tomato paste, stirring until the paste is dissolved. Bring to a boil and skim off any foam that rises to the surface. Reduce the heat to medium low and simmer for 20 to 25 minutes, skimming off the foam from time to time. Strain through a cheesecloth-lined strainer and season generously with the salt.
Make the rice. Bring the broth to simmer in a large saucepan over medium-high heat. Preheat the oven to 500°F. Heat a large flat paella pan or ovenproof skillet over medium heat and add the oil, sea cucumbers, calamari, or octopus, nigella seeds, and the mortar mix. Cook, stirring, for 3 minutes, then add the rice. Cook stirring constantly, umntil the rice begins to color and crackle, 8 to 10 minutes. Add enough of the broth to to cover the rice by about ½ inch (about 3 cups). Boil for 5 minutes on high heat. Add 2 cups of stock, then place in the oven for another 5 minutes, or until the rice has absorbed all the water. It may begin to stick to the pan in some areas, creating a dark brown, crusty layer of rice, which is very delicious if not burned. Remove from the heat, stir once lightly, and allow to settle for a few minutes before serving. Remove the sea cucumbers, calamari, or octoput. Depending on what you are using, slice the sea cucumbers into ¼-inch half-moons and the calamari heads into rings. If using octoput tentacles, cut them in half, if desired. Arrange these over the rice as a garnish, or stir them into the dish fefore serving. Allow about 2 tablespoons of sliced sea cucumbers, calamari, or octopus per serving.