The first thing to do is start caramelizing your onions. Get them going before you even start to prep anything else, as they take a while and you don't want this to hold you up later. Heat a little olive oil in a saucepan and add the onions. Saute until they begin to soften and then add the sugar and garlic. Leave them cooking, covered, on the lowest possible heat, while you prepare everything else. Go in regularly to stir them and make sure they don't stick to the bottom (if they do start to stick, add a little water to the pan to loosen things up)

Peel and chop your potatoes and then add to boiling water in a large saucepan. Simmer for 15-20 minutes, until soft. Use the time they are cooking to grate, slice and crumble your cheeses.

Preheat the oven to 200C / 390F.

Drain the potatoes and add them back into the pan. Add the milk, mustard and butter. Use a hand mixer* to mix everything together until smooth, before adding the eggs and whisking them in too, followed by the cheddar cheese (setting aside a handful of the cheddar for the topping). Add salt to taste. (*It's fine if you don't have a hand mixer, just use a potato ricer or masher and stir in the other ingredients with a wooden spoon.)

Finally, add the caramelized onions in to the potato mixture and stir through. (The onions may not be cooked enough by this point, so if they're still soggy and watery in the pan just turn up the heat, add some extra oil, and get them a little browner and stickier before adding them.)

Pour the mixture into a greased baking dish. The mixture will be runny - that's OK because the eggs will help it to firm up and set when it cooks.

Add the tomato slices, crumbled Lancashire and sliced Goat's cheese to the top of the pie.

Place in the oven and bake uncovered for 30 minutes, until the top is nicely browned.

Let it sit for 5-10 minutes before slicing in and serving it.