I keep it really easy (for the topping). 2parts by wieght of flour, to 1 part sugar and one part butter. I like to use brown sugar, and I often add ground ginger, especially if I am making a rhubarb crumble. Also nice to add crystalised ginger to the rhubarb!
But any fruit is good, so in summer I will use up old fruit, eg nectarines that don't look so pretty but are still perfectly good to eat.
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C and A 9, M 8, S 4!