1small sebago potato (about 200gm), cut into dice5 gm(1¾ tsp) dried yeast230 gmbread flour1½ tbspextra-virgin olive oil, plus extra for drizzling1 tbspthyme, plus extra for scattering20 gmbutter, coarsely chopped1garlic clove, finely chopped2leeks, halved lengthwise and rinsed, cut into 12cm lengths40 gmManchego, coarsely grated
Combine potato in a saucepan with enough cold salted water to cover generously, bring to the boil over medium-high heat and cook until tender (8-10 minutes). Drain well, then mash and set aside to cool.
Combine yeast and 60ml lukewarm water in the bowl of an electric mixer, cover with plastic wrap and set aside in a warm place until foamy (10-15 minutes). Add potato, flour, olive oil, two-thirds of the thyme and 1½ tsp sea salt, mix to just combine, then change to a dough hook and knead until smooth and elastic (3-4 minutes, adding a little extra water if necessary to form a soft dough). Transfer to a lightly oiled bowl, cover tightly with plastic wrap and stand in a warm place until doubled in size (30-45 minutes).
Meanwhile, preheat oven to 180C. Melt butter in a small saucepan over medium heat, stir in garlic and remaining thyme, season to taste and cook gently until fragrant (1 minute). Set aside.
Turn out dough on a lightly floured surface and knock back. Roll out to a rough 15cm x 35cm rectangle and transfer to a large oven tray lined with baking paper. Drizzle with a little extra olive oil, scatter with Manchego and arrange leeks cut-side up on top, pressing into dough. Drizzle with butter mixure, scatter with extra thyme, season generously to taste and stand until it has risen slightly (10-15 minutes). Bake until golden and cooked through (25-30 minutes). Serve warm or at room temperature.