I keep a bag of crumble topping in the freezer

A winter evening, and in the fruit bowl there are three apples nobody wants to bite into any more: a little soft, wrinkled by the stalk. If I had to rub butter into flour from scratch, honestly, I would not bother. But there is a bag of ready-made crumble topping in my freezer, and in half an hour there is a hot pudding on the table. That bag is one of the most useful things I make all year, and in the weeks before Christmas it earns its place twice over.

I make a big batch whenever I have a spare hour. For a batch that does about three crumbles for four people, I use 450 g of plain flour, 250 g of cold butter cut into cubes, 200 g of demerara or caster sugar and 100 g of rolled oats. Some people add chopped nuts or a little ground cinnamon; I keep the basic mix plain and add flavour when I use it, so it suits apples, plums and berries alike.

Everything goes into a large bowl, and I rub the butter into the flour with my fingertips, lifting the mixture as I go, until it looks like rough, damp sand with a few small lumps. The butter needs to stay cold, so I run my hands under cold water first and work quickly. Then I stir in the sugar and oats. A food processor does the same job in a minute with a few short pulses; I just stop before it turns into dough.

Half goes straight onto a crumble for that evening. The rest goes into freezer bags, spread out flat so it does not freeze into a brick, with the air squeezed out. Each bag gets a label: the date and "crumble topping, contains wheat, milk, oats". It keeps for about three months, and because it stays loose, I can take out exactly as much as I need and put the rest straight back.

One thing I do not do is nibble the raw mix. It has raw flour in it, and raw flour can carry bacteria such as E. coli, so it is not for eating until it has been baked. The children know this rule, and they get to lick the spoon from the cooked custard instead.

When I need a pudding, I slice apples into a dish, about 750 g for four people. Pears work too, or a few plums, or a couple of handfuls of frozen berries straight from the freezer. I add a spoonful of sugar, a pinch of cinnamon and a splash of water or lemon juice. The topping goes on straight from the bag, still frozen, in an even layer about 2 cm thick. It actually stays crumblier and crunchier that way.

The dish goes into the oven at 190°C, or 170°C for a fan oven, for 30 to 40 minutes, until the fruit is bubbling at the edges and the top is golden. If I use frozen berries, it needs the longer time, so the fruit is hot right through. The fruit underneath is very hot when it comes out, so I let it stand for five minutes before serving, especially for the children.

I serve it warm, with a spoonful of yoghurt, cream or custard. Leftover crumble goes into the fridge within two hours and is eaten within two days, cold or reheated until piping hot.

At Christmas, a frozen bag also tops mince pies, or goes over stewed apple for a quick dessert when unexpected guests arrive.

Freezer crumble topping. Makes enough for 3 crumbles for 4. About 15 minutes. You need 450 g plain flour, 250 g cold butter, 200 g sugar and 100 g oats. Rub the butter into the flour, stir in the sugar and oats, and freeze flat in labelled bags for up to 3 months. Scatter from frozen over about 750 g fruit and bake at 190°C (170°C fan) for 30–40 minutes.

What fruit goes under your crumble?