1stick unsalted butter, cubed and softened slightly
ice cream, for serving (see headnote)
Position a rack in the upper-middle of the oven and preheat to 375ºPlace a 10" ceramic tart pan (or a 9 or 10" deep dish pie pan or the equivalenon a rimmed baking sheet lined with parchment paper to catch drips.
Make the filling:
In a large bowl, whisk together the sugar, cornstarch and salt. Add the sliced rhubarb, plums and port, and toss to coat. Add the raspberries and fold gently to combine. Scrape the fruit into the baking dish in an even layer. Set aside while you...
Make the topping:
In a large bowl, stir together the flour, oats, brown sugar, cardamom and salt. Add the butter cubes and rub with your fingers until no butter chunks remain and the mixture begins to clump together. Sprinkle the topping over the fruit, using your fingers to break and squish it into roughly almond-sized chunks. Don't pack the filling down.
Bake the crisp: until the juices are bubbling vigorously and overflowing a bit, and the topping is thoroughly golden and cooked through, about 45 minutes. Let the crisp cool and settle a bit, about 20 minutes, then spoon into bowls and top with scoops of ice cream (see headnote for suggestions).
The crisp is best the day it's made, when the topping is, well, crisp, but it will keep for up to a few days and can be reheated in an oven or toaster oven. Store extras in the fridge after 1 day.
Topping is Adapted from Ian Knauer's The Farm via Leite's Culinaria.If you lack quick cooking oats, which are smaller and thinner than old-fashioned rolled oats, David Leite says you can whizz old-fashioned oats in a coffee grinder or food processor to break them up a bit.I haven't tried making this recipe gluten-free yet, but next time I plan to try using oat flour in place of the AP as per my gluten-free Apple Crisple and Strawberry Rhubarb Crumble.Serve bowls of warm crisp with a scoop of vanilla or honey yogurt ice cream.Nutritional values are based on one of eight servings.