Step 1Place the rhubarb, strawberries, zest, sugar, rum or water, tapioca and allspice in a large mixing bowl. Toss them together with your hands until they're well mixed. Let the mixture rest for 20 minutes for the tapioca to soften while you prepare the topping.
Step 1Place a rack in the center of the oven; heat the oven to 350 degrees. Set aside a 9-inch deep-dish pie plate or baking dish on a baking sheet (to catch any juices).
Step 2Place the brown sugar, flour, cinnamon, salt and pecans in a food processor. Pulse on/off to combine. Scatter the butter over the mixture. Pulse on/off until the mixture starts to clump, about 20 to 25 times.
Step 3Transfer the fruit to the pie plate, slightly mounding it in the center. The surface will be uneven. Use your fingers to evenly arrange small clumps of topping over the fruit; gently pat it in place.
Step 4Bake the crisp until the top is well-browned and the juices are bubbling, about 50 to 60 minutes. The crisp needs to rest up to 2 hours before serving. (It can be made a day ahead and refrigerated. To serve, the crisp should be warm. Reheat it in a 350-degree oven until warm, about 10 to 12 minutes.) Serve with vanilla ice cream.