Preparation

• Position a rack in the center of the oven and heat the oven to 350°F. Grease an 8×8-inch Pyrex baking dish with the softened butter.

• Make the topping: In a food processor, combine the flour, brown sugar, oats, cinnamon, and salt and pulse several times to combine. Add the cold butter and pulse until the mixture has the texture of coarse meal and clumps together when squeezed lightly, about 1 minute.

• Make the filling: Combine the rhubarb, brown sugar, cornstarch, lemon juice, lemon zest, and salt in a large bowl and stir with a spatula until evenly mixed. Transfer the rhubarb mixture to the baking pan, and sprinkle the topping evenly over the fruit; the pan will be very full, but the crumble will settle as it bakes.

  Bake until the topping is lightly browned, the rhubarb is tender (probe in the center with a skewer to check), and the juices are bubbling thickly around the edges, 45 to 60 minutes. Transfer to a rack to cool to warm or room temperature and to allow the juices to thicken, at least 1 hour.