Preheat the oven to 350°F. Line a sheet pan with foil and place it on the bottom rack in the oven. Halve and pit the plums then slice into ¼-inch slices. Place the plum slices on several layers of paper toweling in a single layer. Roll the paper towels over the plums into a fairly tight cylinder and set aside to allow the plums to drain a bit.
Remove crust from the refrigerator and unroll. Place the crust into a 9-inch (or 9½-inch but see Café Tips above in the post) tart pan with a removable bottom. Press the crust into the pan to cover and cling to the bottom and sides. With a sharp knife, trim off any excess crust at the top of the pan and discard. Place crust in the refrigerator while you make the filling.
Combine the drained plums, sugar, corn starch, cinnamon and ginger. Stir to combine. Add the raspberries and stir gently. Allow mixture to sit for 5 minutes.
Place the tart (without the streusel topping into the oven right on top of the preheated sheet pan (on the lower rack). Bake for 30 minutes then remove from the oven and add the crumble topping, mounding it just a bit in the center. Return the tart to the oven, on the middle rack this time. Bake for another 20-25 minutes or until the crust and crumble are nicely golden brown.
See Café Tips above in the post for further instructions and tips.