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Position a rack in the center of the oven. Preheat the oven to 200°C. Finely chop 6 garlic cloves.
To a small saucepan set over medium heat, add 6 tablespoons unsalted butter. As the butter begins to melt, add the chopped garlic and 3 oregano sprigs. Cook, swirling, until the garlic is fragrant but not browned and the butter has fully melted, 2 minutes. Remove from the heat.
Using a mandoline or sharp knife, thinly slice 1.5 pounds of Yukon Gold potatoes into ⅛ in-thick rounds.
Using a pastry brush, lightly brush a large rimmed baking sheet with a third of the garlic butter.
Arrange the sliced potatoes in an overlapping shingled manner on the baking sheet until the entire surface is covered. Brush generously with another third of the garlic butter and season well with kosher salt and freshly ground black pepper.
Roast until the potatoes around the edges have begun to turn golden brown, 13-18 minutes. (This will vary based on how thick your potatoes are sliced. As long as the potatoes at the centre of the baking sheet are barely tender at this point, you’ll be in good shape).
Meanwhile, place ⅔ cup of walnuts on a small rimmed baking sheet and toast in the oven alongside the potatoes until golden brown, 8-10 minutes.
Pat dry 1.5 pounds of boneless, skinless flaky white fish with a paper towel. Season all over with kosher salt.
Using a veg peeler, peel 6 large strips of zest from 1 lemon. Set the zest aside. Cut the zested lemon crosswise into thin wheels. When the potatoes are browned at the edges, remove from the oven and arrange the fish in the centre of the potatoes. Brush the fish with the remaining garlic butter. Scatter the lemon slices and the remaining 5 oregano sprigs over the fish. Roast until the flesh of the fish turns opaque and easily flakes away and the potatoes around the edges of the pan are crisp and those at the centre are tender, 15-20 minutes.
While the fish roasts, pick the leaves of 1 bunch of flat-leaf parsley and finely chop. Thinly slice the lemon zest and then chop it finely. Finely chop the toasted walnuts.
In a small bowl, combine the chopped parsley, lemon zest, and walnuts. Grate the remaining 2 garlic cloves into the bowl, season with kosher salt, and toss well with your fingers to combine.
Scatter the gremolata over the fish and potatoes. Squeeze the juice of the remaining lemon over everything and finish with flaky sea salt.