Add your Saffron and chicken if you're making a paella, saffron or not, if you're making a Risotto.
This is basically called a 'sofrito'.......
Add your home made fish stock-strained and tasted (ask the fishmonger, and try to make quite a bit for later with other fish dishes-it can be frozen, even in ice cube trays - head/filleted bones/tails of WHITEFISH like cod and mussel juice, but taste it first for saltiness - cracked shrimp/crab/lobster etc shells, NOT oily fish like salmon/tuna mackerel etc are acceptable - Try 'Googling' fish stock!)
cook, if necessary, the mussels, prawns and other shell fish, keep the juice, but taste it for saltiness (mussels are often extremely salty) - If ok, put the liquids in.......and, if you wish, you can put some chunks of white fish in as well - STIR - but.... for the last time!
Cover with aluminium foil and put into the oven - NO MORE STIRRING!
Leave (check from time to time) for around 30 minutes until the rice is cooked and has absorbed the liquids.
When done, garnish with lemon wedges/parsley sprig...mussels in shells...prawns
Serve with a chilled dry white wine........