Preheat the oven to 220 C.
In a bowl whisk together the crème fraîche, orange zest and juice, vinegar and ¼ teaspoon salt. Transfer half of the mixture to the base of a 23 x 28 centimetre roasting dish and add the tarragon sprigs. Add the chopped tarragon to the remaining mixture in the bowl and set aside in the fridge.
Season the leeks with a ¼ teaspoon of salt and arrange them in the roasting dish, cut side up, cover tightly with foil and roast for 45 minutes until soft, but still holding their shape.
Remove the foil and baste the leeks with some of the roasting juices. Return to the oven, uncovered, for a further 30 minutes, until lightly charred. Remove from the oven and set aside to rest for 5 minutes.
In a bowl combine all the ingredients for the salsa, along with ¼ teaspoon of salt.
To serve, transfer the leeks to a serving platter and pour over the roasting juices. Spoon the remaining creme fraiche mixture over the leeks, followed by the salsa.