Preheat the oven to 375°F (190°C). Set aside a 9x9-inch baking dish or another 2-quart baking dish.
Set aside a small handful of the almonds for topping the crumble. Combine the flour, brown sugar, granulated sugar, remaining almonds, cinnamon, and salt. Stir in the melted butter until the mixture begins to form crumbs. Refrigerate while you make the filling.
To make the filling, assemble, and bake:
Place the sliced pears and plums in a large bowl. Add the sugar, flour, lemon juice, vanilla, and salt. Stir gently until well combined and the fruit is coated.
Transfer the mixture to the baking dish.
Sprinkle the chilled topping evenly over the fruit, gently squeezing some of the mixture together to form larger clumps. Scatter the reserved almonds over the top.
Bake for 45 to 50 minutes, or until the topping is golden brown and the fruit is bubbling around the edges.
Notes
Choosing pears: Bosc and Anjou pears are especially good for baking because they hold their shape well. Bartlett pears can also be used if firm. Choose pears that are ripe but still fairly firm.
Choosing plums: Red or black plums both work well. Choose ripe but still firm plums rather than very soft fruit, which can release more liquid as it bakes.
Adjusting the sugar: If your pears and plums are especially sweet, reduce the granulated sugar in the filling to 1/3 cup (67g).
Fruit size: Fruit varies considerably in size, so the weights are a more reliable guide than the number of pieces. Aim for about 2 pounds of pears and 1 pound of plums before peeling, coring, and pitting.
Storage: Cover and refrigerate up to 4 days. Reheat before serving.