Remove the shells from the shrimp/prawns and keep the shells.
Make the stock by placing the fish bones, shells of the shrimp, celery, carrot, parsley stems, sprig of thyme and a small wedge of the onion in a pot and cover with water. You'll need to add around 4 cups/960ml water to the pot. Bring the pot to a boil and reduce to a simmer (covered).
Meanwhile, finely chop the remaining onion, pepper and garlic.
Heat 2 tbsp olive oil in a large skillet/frying pan over a medium-low heat and fry the fish and shrimp gently until almost cooked through. Remove from the pan and set aside.
Clean out any stuck-on skin etc from pan, add the remaining oil and then the onion and pepper.
Cook for around 10min, stirring now and then (turn lower after a few mins), then add paprika, garlic and fennel seeds.
By now the stock should be ready; strain it to remove all the bones and vegetables and add the saffron to infuse.
Cook the onions and spice mixture for 5 more minutes then add tomatoes.
Cook a further few mins then add fideos (pasta). Stir well so that they are covered by the oily mixture and cook another couple minutes.
Add the stock (3cups/750ml) to the pan, stir then leave to cook at a simmer for 10min.
Stir then add the clams, pushing them under the liquid and pasta. Allow them to cook a minute and add the shrimp/prawns and fish as well. After about 5 mins the clams should open and all should be ready.