2. On a floured surface, roll out the dough 1/8" thick. Lay a bowl about 4 inches in diameter upside down on the dough, and using it as a pattern, cut out 8 circles with a sharp knife. Refrigerate the circles of dough.
3. Wash the rhubarb and trim off any leaves and brown or bruised spots. Using a sharp knife, slice 6 of the stalks on the diagonal about 1/8" thick and place in a stainless-steel bowl. Roughly chop the remaining 2 stalks and keep separate.
4. In a large saucepan, combine the water, sugar and raspberry preserves over medium heat. Bring to a boil and remove from the heat.
5. Pour just enough of the hot liquid over the slices of rhubarb to cover them, reserving about 2 cups of liquid in the saucepan. Keep the rhubarb slices in the liquid until ready to use.
6. Add the chopped rhubarb to the remaining liquid in the pan and simmer until the rhubarb falls apart and the liquid has the consistency of thick tomato sauce.
7. Strain the rhubarb slices and reserve the liquid.
8. Return the liquid to the stove and simmer until reduced to a syrup.
9. Coat several baking sheets with nonstick spray and lay the pastry rounds on them. Spread 1 tablespoon of the thick rhubarb sauce on each round. Arrange the rhubarb slices in a single layer on top of the sauce.
10. Bake the pizzas in the lower half of the oven until the pastry is crisp and golden, about 8 to 10 minutes. Remove from the oven and brush the rhubarb slices with the rhubarb syrup.
11. Top with additional "pizza" toppings or frozen yogurt if desired.