Toast the fideos pasta in a paella pan with a little olive oil over medium heat for 3-4 minutes, stirring often so they don't burn. Remove from the pan and set aside.

Peel the shrimp and use the shells to make a seafood broth. Heat 4 1/4 cups water (1 liter) in a pot and add the shrimp shells. Simmer for 20 minutes, skimming off any foam. Keep the broth warm over very low heat until ready to use.

In the same paella pan used for toasting the pasta, add 3 tablespoons olive oil and sauté the garlic and leek over medium heat for 5 minutes. Add the chopped tomatoes, a pinch of salt, and cook until it forms a paste.

Add the mussels, clams, and chopped squid rings with a little salt. Cook for another 5 minutes over medium heat.

Add the toasted fideos pasta and cook with the sofrito for 2 minutes over medium heat, mixing well.

Preheat the oven to 320°F (160°C).

Pour the hot seafood broth into the paella pan, add the saffron, half a vegetable bouillon cube, and salt to taste. Cook for 8 minutes or until the pasta is al dente.

Place the paella pan in the oven at 350°F (180°C) for 4 minutes to lightly brown the top of the fideuá. It's ready to serve!