For sour cream pastry, process flour, thyme, rind and 1 tsp sea salt in a food processor to combine, add butter and pulse until it resembles sand. Add sour cream, process until mixture just forms a ball, turn onto a lightly floured surface and form into a disc. Wrap in plastic wrap and refrigerate to rest (1 hour).
Meanwhile, place potatoes in saucepan, cover will cold salted water bring to the boil and cook until just tender when pierced with the tip of a sharp knife (15-20 minutes), then drain and set aside to cool slightly.
Melt 20gm butter in a saucepan over medium heat, add leek and half the garlic, and sauté until tender (2-3 minutes). Add wine and half the thyme, season to taste and simmer until wine is completely reduced (2-3 minutes). Set aside to cool.
Preheat oven to 180C. Roll out 8 pieces of pastry on a lightly floured surface to 3mm thick and line the base and sides of eight 11cm-square tart tins or 12cm-diameter tart tins. Trim edges, prick bases with a fork and refrigerate to rest (30 minutes). Blind bake until edges are light golden (5-6 minutes). Remove baking paper and weights, then bake until bases are crisp (2-3 minutes).
Thinly slice potatoes and combine in a bowl with leek mixture and season to taste. Whisk eggs and cream in a separate bowl to combine, and season to taste. Divide potato mixture among tart cases, spreading evenly and adding a few cubes of cheese to the centre of each, then gradually pour cream mixture over, letting it settle
before adding more (there may be a little leftover). Place a slice of cheese on top of each tart, place tart tins on a baking tray and bake, turning tray occasionally, until golden and set (15-20 minutes).
Melt remaining butter in a small saucepan over medium-high heat until it foams, add remaining garlic, hazelnuts and rind, and cook until fragrant and nuts are golden (1-2 minutes). Remove from heat, add remaining thyme, then spoon onto warm tarts and serve.