How to Make It
Preheat the broiler. Roast the red bell pepper under the broiler, turning, until charred. Transfer the pepper to a small bowl, cover with plastic wrap and let steam for 5 minutes. Peel the roasted pepper, discarding the core, ribs and seeds; cut the pepper into 1-inch pieces.
Salt the tuna cubes and let stand for 30 minutes; pat dry with paper towels. In a heatproof bowl, cover the ancho chile with the boiling water and let stand until softened, about 20 minutes; drain. Discard the stem and seeds and coarsely chop the chile.
Bring the wine to a boil in a medium saucepan. Add the mussels, cover and cook over high heat until they begin to open, about 3 minutes. Using a slotted spoon, transfer the mussels to a bowl. Discard the empty shell from each mussel. Season the mussels with pepper and cover with foil; reserve the cooking liquid.
Heat 2 tablespoons of the olive oil in a large skillet. Add the chopped ancho chile and the garlic. Cook over moderately high heat until the garlic is golden, about 2 minutes. Add the tomatoes, pimentón saffron and 1 teaspoon of salt and cook over moderate heat for 5 minutes, stirring. Transfer to a food processor and puree to a coarse paste.
In a medium saucepan, bring the stock to a boil, then cover, reduce the heat and keep at a simmer. In an 18-inch paella pan or a very large skillet, heat the remaining 3 tablespoons of olive oil. Add the tuna and cook over moderately high heat until browned, about 3 minutes. Transfer to a plate and cover. Add the artichokes and scallions to the pan and cook over moderately high heat for 1 minute. Add the ancho-tomato paste and cook, stirring, for 1 minute. Add 4 cups of the stock, the reserved mussel cooking liquid and 1/2 teaspoon of salt and bring to a boil.
Scatter the rice evenly into the pan and stir. Reduce the heat to moderate and cook for 7 minutes. Reduce the heat to low and continue cooking until the liquid has evaporated, about 10 minutes. Add another 2 to 4 cups of simmering stock and cook, shaking the pan and rotating it for even cooking, until the liquid has evaporated, about 10 minutes. Just before the rice is done, during the last 10 minutes of cooking, gently press the roasted red pepper, the mussels and tuna in with the back of a spoon. Continue cooking, shaking the pan, until the rice is just tender but still a bit moist.
Remove the pan from the heat and cover with a towel or paper towels and a foil tent. Let the paella rest for about 10 minutes. Uncover, sprinkle with the parsley and serve with lemon wedges.
Fish fumet is available from specialty food stores.