Step 1Place the flour, sugar, cinnamon and salt in the bowl of a food processor. Process quickly to mix, about 2 seconds. Scatter the butter and pecans over the flour. Pulse the processor on and off until the mixture is crumbly but not massing together, about 20 to 30 seconds.
Step 1Heat the oven to 350 degrees. Set aside a 13x9-inch baking dish.
Step 2Place the rhubarb, strawberries, sugar, tapioca, juice, zest, vanilla and allspice in the baking dish. Use your hands to toss until well mixed. Let it rest 20 to 30 minutes, stirring once midway. Be sure the fruit is evenly spread in the dish. Sprinkle the topping evenly over the fruit. Place the pan on a large baking sheet or a large sheet of heavy-duty foil to catch any bubbling juices.
Step 3Bake until the top is browned and the juices are bubbling, 65 to 70 minutes. Let cool at least 1 hour before serving. (Alternately, cool completely. The crisp can be refrigerated overnight, covered airtight. To serve, let come to room temperature, uncovered. Reheat, uncovered, in a 350-degree oven until warm, about 12 minutes.) Serve warm with small scoop of ice cream.