After patting them dry, generously season fish fillets with salt and pepper. Cut fillets into 2-inch pieces and set aside.
Make bouquet garni…Place parsley stems in a sachet or the center of a cheesecloth along with fennel seeds and bay leaf. Tie up the bag and make a knot or gather the sides of cheesecloth and tie it up with kitchen twine; set aside.
In a Dutch oven over medium, heat oil. Add leeks, onions, celery and carrots; season with sea salt. Cook veggies for 10 minutes, stirring often.
Add pressed garlic and sauté for 30 seconds. Add tomatoes, white wine and season with black pepper. Stir well and cook until the liquid has reduced by half.
Add potatoes, chicken broth, bouquet garni and piment d’Espelette. Stir and bring the mixture to a boil. Cover, reduce back to medium and cook for 10 minutes stirring occasionally.
Pour in clam juice and add seasoned fish fillet pieces; push down to submerge the fish. Turn the heat to high and when it just starts bubbling, turn back to medium and simmer for 10 minutes.
Meanwhile make the Aïoli...
Using a slotted spoon, transfer fish pieces to a plate, remove the bouquet garni and discard it.
Check if potatoes are tender – if not, cook for 2 minutes longer.
Temper the garlic sauce by pouring in some of the hot fish liquid, in a slow stream, while whisking continuously before adding more until the Aïoli is tempered.
Add it to the mixture and whisk until well incorporated
Spoon soup in warm bowls, place a few fish pieces on top (and clams if using them), garnish with fresh chopped parsley and serve with crostini or crusty bread.
Notes
Footnote: Substitute cayenne pepper but only add 1/2 tsp. instead of 1 tsp.