Heat olive oil in a 15-inch paella pan over medium heat. Sauté onions until translucent (about 5 minutes). Add garlic and cook for 1 minute until fragrant.
Add diced tomatoes and bell peppers. Cook for 5-7 minutes until vegetables are soft and most liquid has evaporated.
Stir in the rice, coating each grain with the sofrito mixture. Toast for 2-3 minutes until the rice becomes slightly translucent.
Add paprika and crumbled saffron threads. Pour in the seafood stock. Bring to a simmer and season with salt and pepper.
Simmer for 18-20 minutes until rice is almost done. If using frozen peas, add them now.
Arrange seafood on top of the rice, starting with fish, then shrimp, and finally mussels. Push them slightly into the rice.
Cook for an additional 8-10 minutes until seafood is done and mussels have opened.