Heat the oven to 375°F (190°C). Butter the dish and place it on a rimmed sheet to contain fruit juices if they bubble over.
Pit and slice plums, leaving the skins on for color. Toss with fruit sugar, cornstarch, and vanilla until evenly coated.
Spread fruit in the dish, keeping the depth similar across the pan. Include the coating and juice from the bowl.
Combine all-purpose flour, almond flour, brown sugar, and salt. Cut in cold butter until uneven crumbs form, then fold in sliced almonds.
Scatter topping loosely across the plums. Keep a few small openings so steam can escape and you can see central fruit juices.
Bake 35–45 minutes until golden and bubbling near the middle. The plums should be soft when pierced with a thin knife.
If the almonds color quickly, tent loosely with foil. Continue baking until the fruit beneath the center topping is cooked through.
Rest at least 25 minutes before serving. Spoon into bowls with a little cream if desired, remembering that this crumble contains almonds.
