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Heat the butter in a large pan. When foaming, add 2 tbsp olive oil and the onions. Cook over a medium-low heat until softened and golden, about 30 mins. When the onions have softened, stir in the garlic and thyme. Season and set aside.
Heat oven to 200C/180C fan/gas 6. Bring a large pan of salted water to the boil, add the potatoes and boil for 5 mins before draining well.
Mix the drained potatoes with the remaining oil and some seasoning in a large roasting tin. Roast for 30 mins, turning halfway, until golden. Stir in the onion mixture and roast for 10 mins more until hot.