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.