In a pan, melt a tablespoon of the butter. Add the fennel slices and sprinkle a pinch of salt. Let cook for a couple of minutes. Then add the wine and let cook till the fennel softens. Remove from the heat and in a blender/food processor, blitz till the mixture is smooth.
Return the fennel puree to the heat and add the chicken stock and thyme. Check for seasoning, adjust and let simmer slowly for about 10 minutes.
Remove from heat. Stir in half of the grated lemon zest and top the sauce with some finely chopped fennel fronds and the rest of the lemon zest