Put the oats, if using, in the bowl of a food processor and pulse until they’re ground almost to a flour (but not quite – a bit of texture is nice).
Add the flour, salt and sugar and pulse to combine. Drop in the butter and sour cream and run the processor just until the pastry comes together in a few clumps. Tip it onto a large piece of cling film and bring the dough together with your hands, pressing it into a disc about 2.5cm/1” thick before wrapping in the cling film and setting aside in the fridge for at least half an hour or up to a day.
When you’re ready to cook, set your oven to 400⁰F/200⁰C on combination steam setting. If your oven has variable steam settings, use 50%. If not, don't worry! Just set to combi steam at the correct temperature and the oven will figure out the humidity for you. Line a dark coloured baking tray with baking paper and set aside.
Remove the pastry from the fridge, place it on a flour-dusted bench and roll it into a rough circle about 25cm/10” diameter. Don’t worry if the edges crack and it’s not completely even, you won’t notice this later. Transfer the pastry to the lined baking tray.
Sprinkle the breadcrumbs over the pastry, leaving a 5cm/2” border all the way around (the crumbs are there to soak up any excess juices from the fruit which might stop the bottom of the galette cooking properly). Pile the cut plums over the top of the breadcrumbs and sprinkle with the sugar, then gently lift and pleat the pastry around the fruit to encase everything. If you get any tears or holes just squash it back together or patch with a little piece of pastry torn off the top edge. You could glaze the pastry edges with egg yolk at this point, though I don’t usually bother.
Cook the galette for 25-30 minutes, until the fruit is bubbling and collapsed and the pastry is golden brown. If your galette is like the one I photographed, you might have a couple of spots where the juices burst through the pastry. I don’t worry too much about that – the juice just pools around the edge of the galette a bit but shouldn’t affect the base. Remove from oven and allow to cool until warm before serving with cream or ice cream, if you like. Any leftovers are delicious cold or can be reheated in your steam oven the next day.