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1
Turn the oven to 350F (176C). While the oven is heating up, spread the walnuts across a baking sheet. Bake the walnuts for 5 minutes and then remove the baking sheet from the oven; allow the walnuts to cool.
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2
Move the cooled walnuts into the bowl of a food processor. Pulse them, about 50 times, until you have a dusty walnut crumble. Then add the: flour, light brown sugar, butter (cut into pieces), lemon zest, and pinch of salt to the bowl. Mix all of the ingredients together for about 45 seconds, until it is just shy of doughy. Break up the crisp topping across a baking sheet. Move it to the freezer while you wash the dishes and prepare the other ingredients.
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3
Heat the oven to 375F (190C). In a large bowl, stir together the: frozen raspberries, frozen Italian plums, lemon juice, vanilla powder, light brown sugar, and (if you are using it) the cornstarch. Butter the inside of a 10-inch pan. (You can also use a casserole dish, but I like more surface area, for the extra caramelization.)
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4
Pour the fruit into the buttered pan, and then scatter the chilled crisp topping all over the fruit. Move the pan to the oven. Bake for 45 minutes (if the crisp is coming from the freezer, add an extra 5-10 minutes to the cooking time), or until the crisp topping is golden and the fruit is bubbling. Allow the crisp to cool for 20 minutes before enjoying.
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5
(You can make the crisp, cover it well with plastic wrap, and store it in the freezer for up to 1 month-it goes directly from the freezer into the oven, making it the perfect, simple solution for house or dinner guests.)
