In a 15-inch paella pan, large, deep cast-iron skillet, chicken fryer, or Dutch oven warm the oil over medium heat. Add the onion, bell pepper, and garlic and sauté until softened, 2 to 3 minutes. Add the rice, 1 teaspoon salt, and the smoked paprika and cook, stirring, to slightly toast the rice, 1 to 2 minutes.
Bring the stock to a boil and stir the saffron into it. Stir the tomatoes, half the drained roasted red peppers, and the stock into the rice mixture, spreading the rice out evenly in the pan. Bring the mixture to a boil, and cook over medium heat, uncovered, for 10 minutes. Do not stir the rice after this.
If using littleneck clams, arrange them around the edges of the skillet, spacing them evenly. Cover with a tight-fitting lid or with foil, sealing it tightly, and cook for 2 minutes.
If using smaller clams, working from the outside of the skillet in, arrange the mussels between the clams around the edge of the pan and the rest in a ring inside them, followed by the shrimp. Sprinkle the seafood with 1 teaspoon salt. Reduce the heat to low, cover the pan, and simmer (without stirring) for 15 minutes, or until the rice is tender and the clams and mussels have begun to open. Arrange the lobster, if desired, in the center of the pan, cover, and cook until the lobster is heated through and the clams and mussels are fully open, about 5 more minutes. Discard any clams or mussels that don’t open. Turn off the heat and let the paella rest on the burner for 8 minutes to set it.
Decorate with the remaining sliced roasted red peppers. Arrange any separately cooked seafood over the top and serve from the pan with lemon wedges on the side.