Arroz de Marisco, similar to Spanish Fideuà, focuses on maximizing the flavour of the broth. The secret lies in slowly cooking the onion, garlic, and tomatoes (sofrito), which creates the foundation for the liquid and rice.
Unlike Italian risotto, the Portuguese version is not stirred as frequently and is intentionally 'wet' or 'soupy' – the rice should not be mushy but should float in a flavourful sauce. Coriander (cilantro) is key, but if you dislike it, use parsley.
Thoroughly clean the mussels and clams. Discard any that are open and do not close when tapped.
In a large pot or wok, heat 1 tbsp of olive oil. Add the shellfish and pour in the white wine. Cover and cook until all the shells open (about 3-5 minutes).
Remove the shellfish with a slotted spoon (discard unopened ones!). Strain the cooking liquid through a fine sieve (or coffee filter) to remove sand. Remove some shellfish from their shells for easier eating.
In the same pot, heat the remaining olive oil. Sauté the onion until translucent. Add the garlic and tomatoes. Cook for about 10 minutes until a thick tomato sauce (sofrito) forms.
Add the rice and sauté for one minute to coat the grains in the oil and sauce.
Pour the strained shellfish liquid and fish stock over the rice. Bring to a boil, season with salt and pepper.
Cook the rice over medium heat for 15-20 minutes. Do not stir too often; only stir gently occasionally to prevent the rice from sticking. The goal is to get creamy rice in a broth-like sauce, not a dry risotto.
The rice should be tender but still have a visible amount of sauce.
About 2-3 minutes before the end of cooking, add the peeled prawns (and any other quickly cooked seafood). Heat them briefly until they turn pink.
Stir in most of the coriander and the cooked shellfish. Remove from heat and let rest for 5 minutes.
Serve immediately in deep plates, sprinkled with the remaining coriander and with bread for dipping into the sauce.