Combine flours, thyme, yeast and 3 tsp salt in an electric mixer fitted with a dough hook and mix to combine. Whisk 100gm sour cream and 350ml lukewarm water in a bowl to combine, then, with mixer on medium speed, gradually add to flour mixture and knead until smooth and elastic, and dough pulls away from the sides of the bowl (5-6 minutes). Transfer to a lightly oiled bowl, cover and set aside until doubled in size (1 hour).
Meanwhile, for kale filling, melt butter and oil in a saucepan over medium-high heat, add leek and garlic and sauté until tender (4-5 minutes). Add kale, stir occasionally until wilted (2-3 minutes), then stir in thyme, chilli and lemon rind. Remove from heat, season to taste and set aside.
Preheat oven to 200C, and butter and line the base of a 20cm x 30cm baking tray with baking paper. Turn out dough onto a lightly floured surface, knock down, then roll out to a 30cm x 45cm rectangle, with the longest side facing you. Spread with remaining sour cream, scatter with cheese and kale filling, then season to taste. Roll into a cylinder, trim uneven ends, then cut into 12 even pieces. Place cut-side up in prepared tin, cover and stand until risen by half (10-15 minutes). Bake, turning tray occasionally, until golden brown (30-40 minutes). Stand in tray for 10 minutes, then serve warm.