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Heat the oil in the Dutch oven until hot, add onions, and garlic and sweat until fragrant, making sure they don't pick up much color, 3-5 minutes.
Add celery, carrots, fennel and saffron and continue sweating until fragrant, but not soft, 2-3 minutes.
Add potatoes, stock, tomatoes, stirring to combine.
Turn the heat down to a simmer and reduce until thickened, stirring occasionally. This should take about 20-30 minutes. Season to taste with salt and pepper.
When the soup has thickened to stew consistency, add the shrimp, followed by the clams and mussels.
Cook until the clams and mussels are opened, about 3-4 minutes. Stir in the scallops and turn off the heat. Stir in Pernod and adjust seasoning to taste.