In a paella pan or a large cast iron skillet, heat vegetable oil until hot on medium-high heat. Saute onion, garlic, and parsley until onions are softened slightly and garlic is fragrant, then add chopped tomatoes and season lightly with salt and pepper. Continue to fry until tomatoes caramelize slightly (about 5 minutes), then stir in paprika and rice.
When rice is thoroughly mixed in with veggies, stir in chicken stock, chorizo sausage, and saffron threads, spreading rice mixture into an even layer on the bottom of the pan. Wedge clams and mussels into rice layer, and lay prawns atop rice; bring stock to a boil, then reduce heat and cover pan tightly. Cook on low heat for about 15 to 20 minutes or until rice is fluffy and tender. Turn heat up to medium for final minute to form the prized crust on the bottom of the pan, then serve with lemon wedges!