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1
Pre-heat the oven to 180 degrees.
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2
Peel the spuds and cut them into slices about 3mm thick.
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3
Drop these all together into a big pan of water at the boil and cook for no more than 3 minutes, then drain them.
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4
In a frying pan over a medium heat cook the onions and bacon together in the oil for about 5 minutes to soften the onion and colour the bacon, adding the garlic for the last minute.
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5
Seasoning lightly with salt and pepper as you go put half the onion and bacon mix in the bottom of a flat gratin dish; cover with half the potato slices, roughly overlapping; then another layer using the rest of the bacon and onions; then the last of the potato slices.
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6
Top with the grated cheese and the butter cut into little dice, then pour over the milk (you can warm it briefly in the microwave to give it a start) which should come almost up to the top - you want a bit of potato and all the cheese to be above the milk.
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7
Put the gratin dish (best to have it within a big roasting dish in case of spills) in the pre-heated oven, and start checking it after 30 minutes - the cooking time depends on numerous factors, but it's very forgiving.
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8
It's ready when the cheese is melted and browning, and the potatoes accept the tip of a knife easily.
