Combine the stock and squid ink in a saucepan and place over medium-low heat, stir to combine.
Heat 3 tablespoons of the olive oil in a wide shallow skillet or paella pan over medium heat. Make a sofrito by sauteing the onion, garlic, and tomatoes; cook until the mixture caramelizes a bit and the flavors meld, about 10 minutes. Sprinkle with the paprika, salt and pepper. Fold in the rice, stirring to coat the grains. Pour in the hot, black stock and give it a stir. Reduce the heat to low and cook until the liquid is absorbed, about 10 minutes; season with salt and pepper. Do not cover or constantly stir like risotto.
Mix the pimentos, olives, parsley, lemon juice, and remaining 2 tablespoons of oil together in a small bowl. Arrange the pan-roasted prawns on the black rice, and sprinkle the olive mixture all over the top; serve directly from the pan at the table.
Make a quick marinade for the prawns by combining them in a bowl with a healthy drizzle of olive oil, the paprika, crushed garlic, lemon slices, salt, and pepper. Toss the prawns so they are coated with all the good stuff and marinate for 5 minutes to 2 hours in the refrigerator.
Place a large skillet over medium-high heat and coat with a couple of tablespoons of olive oil. When the oil is hot, pull the prawns out of the marinade and toss them into the pan. Saute for 3 minutes on each side until the shells are red and charred. Serve the prawns with the Spanish black rice.