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Preheat the oven 400 degrees. Spray a 9-inch square baking pan with cooking spray and set aside.
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Cut the butter into 1/4-inch cubes and refrigerate while you make the filling.
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Combine the apples, rhubarb, lemon juice, vanilla, cinnamon, and 2 tablespoons of the brown sugar in a medium mixing bowl. Stir until the fruit is evenly coated with the sugar.
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Spread that mixture into an even layer in the prepared baking dish.
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Combine the oats, almonds, salt, and remaining brown sugar in the bowl of a food processor. Remove butter from the fridge and sprinkle it over the top. Pulse 10 to 20 times, until combined but still pretty chunky.
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Sprinkle the topping evenly over the fruit.
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Set the pan on a large baking sheet to catch any dripping juices. Bake 30 to 40 minutes, until the topping is browned and the juices are bubbling.
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Cool at least 20 minutes, then serve.
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Store uneaten crisp loosely covered in the fridge for 2 days. (Don’t cover the crisp until it’s fully cooled or it could sweat.)