In a large pot drizzle some olive oil and add all the cleaned mussels and clams, making sure to throw away the cracked ones as they may have sand inside.
Turn on the heat to medium and cover the pot with the lid to steam the mussels and clams. Stir a couple of times and once they are all open, turn off the heat and set aside. (If you use frozen seafood, start from next step)
In another large pan, drizzle some more olive oil, add the 2 finely chopped shallots, minced garlic, and the tomato paste. Turn on the heat to medium and cook for a couple of minutes.
Add the rice to the pan and toast for another couple of minutes (be careful not to burn the onion and garlic.
Pour the white wine and let it evaporate for a minute, then add the fish stock a couple of ladles at a time until completely absorbed. Cook for about 20 minutes, adding more stock when necessary.
Halfway through the cooking time of the rice, add the mussels' water instead of the stock (if it's becoming too salty, add plain water). Add the cleaned and deveined shrimps and the cleaned squids, cut into rings.
When the rice is done, turn off the heat and add the butter, mussels, clams, the finely chopped parsley and the chili pepper flakes before serving.
Notes
For the best risotto alla Pescatora, use the freshest seafood you can find. Fresh ingredients make a significant difference in the flavor and overall quality of the dish.