The beautiful tomato and fennel garnish is what takes most of the time to prepare in this otherwise quick dish, but you can prep the vegetables ahead of time; keep them refrigerated separately. Take them out of the refrigerator as you start the leeks to let them warm up a bit and combine them just before serving.
Cut the leeks in half lengthwise, and then into a fine julienne. Rinse well in cold water and dry. Remove and reserve the celery leaves from the stalk before trimming the stalk and slicing it thinly. Slice the fennel bulb as thin as possible, preferably on a mandolin, and then cut those slices in a thin julienne. Quarter the tomatoes, scoop out the seeds and pulp, and chop the flesh into a tiny dice. (The celery and leeks get cooked with the mussels; the fennel and tomatoes go on after as garnish.)
Heat a large pot over medium heat. Add the butter and 2 tablespoons of olive oil. Add the leeks, celery, and garlic and cook until softened but not browned, about 7 minutes. Crumble the saffron threads info the pot. Add the red pepper flakes, wine, 1/2 teaspoon of salt, and the mussels. Increase the heat to medium-high, stir to distribute the flavorings, cover the pot and cook, stirring again after after a couple of minutes, until a peak inside the pot shows that the mussels have all opened, 7 to 10 minutes. Remove the pot from the heat and, using a slotted spoon, transfer the mussels and leeks to a large bowl, leaving any liquid behind in the pot. Cover the mussels to keep them warm while finishing the sauce.
Pour the cooking liquid into a bowl or measuring cup, leaving any sediment behind. Wipe the pot clean, and return the heat to medium-high and whisk in the crème frâiche (or the heavy cream and lemon juice). Reduce to a simmer and cook the sauce until hot, about 2 minutes. Remove from the heat and season to taste with salt and pepper.
Just before serving, combine the fennel, tomato, and any reserved celery leaves with 1 tablespoon of olive oil and the lemon juice. Season to taste with salt and pepper.
Put the mussels into the pot to reheat them briefly; if the sauce is still not very hot, heat it for a minute on the stove.
Divide the mussels among six pasta bowls( or put them into one large bowl and have people serve themselves.) Pour any crème frâiche sauce left in the pot over the mussels and top them with the tomato-fennel mixture.