Heat the olive oil in a paella pan or a wide, shallow skillet over medium heat.
Add the diced onion and red bell pepper, sautéing for 4-5 minutes until the vegetables are softened and fragrant.
Stir in the minced garlic and smoked paprika, cooking for 1 minute to release the oils, then add the dry rice and stir to lightly toast the grains.
Add the tomato puree and saffron threads, stirring to combine with the rice mixture.
Pour in the seafood stock and bring to a gentle simmer; spread the rice into an even layer and do not stir again to allow the 'socarrat' (crispy bottom) to develop.
After the rice has simmered for about 10 minutes and some liquid has absorbed, nestle the shrimp, mussels, calamari rings, and peas into the rice.
Cover the pan with a tight-fitting lid or foil and cook for an additional 8-10 minutes, or until the rice is tender, the shrimp are pink, and the mussels have opened.
Remove from heat, season with sea salt and black pepper, and let rest for 5 minutes before garnishing with fresh parsley and a lemon wedge.