For the raspberry syrup: Combine the raspberries, lemon juice and 1/3 cup water in a large metal bowl. Crush the berries and set the bowl over a pot of simmering water, double-boiler-fashion. Cook very gently until the raspberries begin to dissolve, about 15 minutes.
Pour the mixture into a jelly bag (or a large sieve lined with two layers of cheesecloth) set over a deep bowl. Let the mixture drip, gently stirring the pulp now and then but not pressing on it, until it seems that all of the liquid has been expelled, 1 to 2 hours.
Measure the juice, pour it into a wide-bottomed saucepan and then measure out an equal volume of sugar (for example, for 1 1/2 cups of juice, you'd need 1 /¿2 cups of sugar). Add the sugar to the juice and set the pan over medium-high heat. Bring the syrup to a rolling boil (watch to make sure it doesn't boil over), boil for 30 seconds and then remove from the heat. Let it settle for a minute, and then carefully skim off the foam. Bring to a rolling boil again and then immediately remove from the heat. Let it settle, and skim it again. Measure out 1/2 cup of the syrup for use in the baked rhubarb and set aside; refrigerate the rest of the syrup for another use.
For the baked rhubarb: Preheat the oven to 325 degrees F.
Trim and discard the ends of the rhubarb; cut the stalks into 3/4-inch-thick slices on a deep diagonal, keeping them neatly stacked together. Transfer the slices to a 9-by-13-inch baking pan, making rows of cut rhubarb, until you have filled the pan.
Warm the reserved raspberry syrup in a microwave oven (or a small saucepan) until hot. Add the sugar to the warm syrup and stir to dissolve. If using the vanilla bean, slice it lengthwise, scrape out the pulp and throw the pulp into the syrup and the pods in with the rhubarb. If using vanilla extract, add it to the syrup.
Pour the raspberry syrup evenly over the rhubarb and bake, uncovered, until the rhubarb is soft but not mushy, 15 to 18 minutes. Halfway through baking, remove the pan from the oven, and gently tilt the pan and spoon the syrup over the rhubarb to glaze it.
Allow the baked rhubarb to cool in the pan, glazing it once more with the syrup as it cools. Refrigerate the rhubarb in the pan. (The rhubarb will keep in the refrigerator for up to 1 week, and gets firmer and brighter as it sits.)
Serve over vanilla ice cream.
Recipe courtesy of Amy Thielen, reprinted from The New Midwestern Table. Copyright 2013 by Amy Thielen. Photographs 2013 by Jennifer May. Published by Clarkson Potter, a division of Random House LLC.