Clean the clams thoroughly. Cook them over high heat in 50 cl water until they open, then shell and reserve the meat. Strain the cooking juices to remove any sand.
Finely chop 2 garlic cloves and 1 onion (noted as leek in original, but context suggests onion; clean and julienne the leek). Blanch tomatoes for 2 minutes, peel, seed, and dice. Peel and dice the potatoes.
In a saucepan, gently sauté the onion in 2 tbsp olive oil without browning. Add garlic, tomatoes, potatoes, white wine, clam cooking juices, thyme, bay leaf, 1 pinch saffron, and shellfish fumet. Season with salt and cayenne. Simmer for 30 minutes, adding clams 5 minutes before end.
For the rouille: Crush 1 garlic clove with salt and cayenne in a mortar. Add 1 pinch saffron and egg yolk, then emulsify with 15 cl olive oil like mayonnaise. Set aside at room temperature.
Blanch leek julienne in boiling salted water for 5 minutes, drain, and shock in ice water to preserve color.
Discard thyme and bay leaf from soup. Reserve 12 clams for garnish, blend the velouté smooth. Stir in leeks and simmer 5 more minutes.
Adjust seasoning, stir in cream and remaining clams. Garnish with chives and serve hot with rouille on the side.
