Step 1

Put the sliced onions, olive oil and half the butter in your largest frying pan. Cook over a low-medium heat until the onions are super-soft but not golden-brown yet – this might take up to 30–35 minutes.

Step 2

Meanwhile bring a large pan of salted water to the boil. Slice the potatoes into 1cm/½ in thick slices. Once the water is boiling, add the potatoes and when the water has returned to the boil, set your timer for 3 minutes. Drain immediately and leave in the colander to steam dry. Heat the oven to 200C/180C/Gas 6.

Step 3

When the onions are done, remove from the heat and stir through the parsley and thyme with the garlic and some generous seasoning. Gently toss the onion mixture and par-boiled potatoes together – you can do this in the frying pan if they fit, or a large mixing bowl.

Step 4

Tip everything into a large baking dish – you don’t want the mixture to sit any deeper than 5cm/2in. Arrange and press the top down a little to roughly flatten. Dot the remaining butter all over the top.

Step 5

Bake for 35–40 minutes, or until the top is golden-brown and crisp, and the potatoes cooked through. A little extra garnish of chopped parsley and fresh thyme is nice, but not strictly necessary.