Gently place saffron threads into large, heavy saucepan and cook until it gives off an aroma
Add the fish stock to the pot, bring to a boil, then reduce heat to simmer to infuse the saffron flavour
Heat olive oil in a large skillet and add onions and garlic - stir until onions start to soften
Add shrimp, oysters, and red peppers to skillet and cook for about 2 minutes, stirring frequently
Pour rice into the skillet, and stir thoroughly to ensure all the grains are well coated with oil
Add half of the simmering fish stock to the skillet, stir thoroughly and then bring to a boil. Reduce the heat and simmer until all the liquid is absorbed - about 5 min
Transfer the rice mixture into the large sauce pan with the remaining fish stock
Gently stir in the tomatoes and peas, and stir until evenly mixed
Arrange clams and mussels on top of the mix and simmer another 10-15 minutes until liquid absorbed
Cover pot and let stand for at least 5 minutes
Remove any clams or mussels that didn't open, and stir the paella gently to fluff
Sprinkle chopped parsley over top of the paella and serve