Combine the oatmeal, sugar, walnuts, flour and cinnamon in a large mug. Using a fork, cut in the butter until the texture becomes crumbly.
Place the berries on top of the crumble mixture and microwave on high power for 2 minutes, until the fruit starts to bubble and slightly thicken. Stir to bring the crumble to the surface and serve with a dollop of yogurt on top.
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