In a 4 cup food processor or larger, combine all of the tart dough ingredients and pulse until the mixture is coarse and sandy. The dough will start to form together in a ball.
Remove from food processor and spread the dough with your hands into a 9 inch tart pan. Press the dough into all the ridges and make sure to evenly spread the dough out. Sometimes I will use the bottom side of a measuring cup to help press the dough. Remove any excess dough that has gone over the top of the pan. Using the tines of a fork, poke a handful of holes into the crust to allow for air to escape.
Place the tart pan with dough in a refrigerator for 20 minutes. This would be a great time to cut your rhubarb and decide what decoration you would like to make, if any.
Remove the tart dough from the refrigerator and gently place a piece of parchment paper over the top and weigh down with pie weights. We are going to partially bake the crust using a blind-bake method. (NOTE: aluminum foil is a great substitute for parchment paper. If you do not have pie weights, dried beans or rice would be a perfect substitute.)
Bake the crust for 10-15 minutes, or until the edges barely begin to turn brown. Remove from oven and begin preparing the frangipane. Keep your oven on at 350° F.