In a paella pan, heat olive oil over high heat, and sear the shrimp and scallops for 1 minute on each side.
Remove from the pan and set aside.
In the same pan, sauté onions and garlic until softened and aromatic.
Add diced green and red bell peppers, cooking until tender.
Stir in tomato paste and cook for 2 minutes.
If using saffron, infuse it in a small amount of warm water and add it to the pan.
Add white rice to the pan, stirring to coat each grain.
Sprinkle smoked paprika evenly over the rice and pour in fish stock.
Add the tomato puree and stir to combine.
Season with cayenne pepper, salt, and black pepper to taste.
Bring to a boil.
Nestle shrimp and scallops into the rice, distributing them evenly.
Turn down the heat to low, and simmer the paella for 10 to 15 minutes, or until the seafood is cooked through and the rice is tender.
Rest the paella for a few minutes before serving.