Heat the olive oil in a large pot or skillet over medium heat. Add potato, garlic, leek and carrot cubes and sauté until just softened and slightly brown, for about 5 minutes.
Add in lemon zest and thyme, followed by tomatoes and saffron.
Stir all the ingredients then pour in the fish stock.
Bring to a boil and simmer until the veggies are tender and stock is reduced by half, about 15 minutes. Season to taste with sea salt and black pepper.
Reduce heat to medium-low and fold in the fish chunks. Cook for a couple of minutes.
At this point, add in any seafood using: clams, mussels, and shrimps. Simmer until the clam/mussel shells open, about 5 minutes more.
Sprinkle with parsley and extra lemon zest if you like, and serve with croutons or grilled bread slices.