Combine the potatoes and enough cold salted water to cover generously in a large saucepan, bring to the boil and cook until tender when a skewer is inserted (15-20 minutes). Drain, return the potatoes to pan, mash with a masher or pass through a ricer and keep warm.
Meanwhile, melt the butter in a saucepan over medium heat, add the leek and sauté until starting to soften (4-5 minutes). Add the thyme and oregano, sauté for another minute, then set aside.
To make the béchamel, melt the butter in a saucepan over medium heat, then when foaming, add the flour and stir continuously with a wooden spoon or whisk until sandy textured and golden (2-3 minutes). Gradually add the milk, whisking continuously until smooth before adding more, then whisk continuously until thickened and mixture comes to the boil (8-10 minutes). Whisk in edam cheese to combine, then season to taste with sea salt and freshly ground black pepper. Add to the mashed potato, then add the leeks and stir in crème fraiche and jamon and adjust seasoning to taste. Spoon into a piping bag fitted with a 2cm plain nozzle.
Lay a 40cm long piece of plastic wrap on a work surface and pipe croquette mixture along the middle to form a 25-30cm length about 2.5cm in diameter, then carefully roll up in the plastic wrap to form a cylinder. Tie a knot in each end of the plastic wrap, place on a tray and freeze until quite firm (30-40 minutes
Meanwhile, make the chipotle mayonnaise by mixing the ingredients in a bowl to combine, season to taste and set aside until ready to serve.
Heat the oil to 170°C in a large heavy-based deep-sided frying pan or deep saucepan. Unwrap croquette from the plastic wrap and cut into 6cm lengths. Place the flour, egg and panko crumbs in separate bowls, then dip each croquette in flour, then egg, then crumbs, shaking off excess in between, and place in a single layer on a tray. Fry croquettes in batches, being careful not to overcrowd the pan, turning occasionally until golden brown (3-4 minutes, be careful as the hot oil may spit). Drain on paper towel for 1 minute, then serve hot with chipotle mayonnaise.