Step 1Heat the oven to 375 degrees. Use 1 tablespoon butter to generously grease the bottom and sides of a 13-by-9-inch baking dish.
Step 2Combine the flour, brown sugar, pecans, oats, cinnamon, ginger, allspice and salt in a bowl. Add the remaining butter and mix with your fingers or a wooden spoon until well blended.
Step 3Combine the rhubarb, sugar and cornstarch in a second bowl and mix well. Spread evenly into the prepared pan. Crumble the topping evenly over the rhubarb.
Step 4Bake until the rhubarb is tender and the topping is crisp and browned, about 30 minutes. Serve hot or warm, with ice cream.