Step 1Heat the oven to 350 degrees. Set aside a 9-inch deep-dish pie plate or baking dish on a baking pan (to catch any juices).
Step 2Place the plum slices and raspberries in a large bowl. Add the zest, orange juice, flour, sugar and tapioca to the fruit. Toss until well mixed. Let rest 30 minutes, tossing several times.
Step 1Place the butter, brown sugar, flour, cinnamon, salt and pecans in the bowl of a food processor and process until the mixture starts to clump. Set aside.
Step 2Transfer the fruit mixture to the reserved dish; level the surface. Use your fingers to break up large clumps of topping (if any) and scatter evenly over the surface of the fruit. Gently compact the topping with your hands.
Step 3Bake the crisp until the top is brown and the juices are bubbling, about 1 hour 5 minutes. The crisp needs to rest 2 to 3 hours before serving (to let the juices thicken). (It can be made a day ahead and refrigerated after it's completely cooled, loosely covered with foil.)
Step 4Serve warm. Reheat in 350-degree oven until warm, about 12 to 15 minutes. Serve with ice cream.