To prepare the crust, mix together the dry ingredients in a large bowl. Grate the frozen butter over the bowl on the largest hole setting, stopping every couple minutes to stir in the grated butter bits so that they don’t form one giant butter shard clump. In a small bowl, mix together the vanilla extract with the 1/4 cup water. Add the water mixture in small increments, stirring, until all the water is added. Knead the dough for about 30 seconds to help disperse the moisture, then grab a handful of the mixture and squeeze. If it generally sticks together when you let go, it is fine. If it completely crumbles apart, it needs another tablespoon or two of water. Once the dough holds its shape, cover with plastic wrap and place in the refrigerator.

Preheat the oven to 375 degrees Fahrenheit. To prepare the filling, gently toss together the peaches, plums, rhubarb, and vanilla extract. Add the remaining ingredients and toss until the fruit slices are coated in the sugar and spice mixture. Set aside.

Roll out the dough to 1/4-inch thickness on a lightly-floured surface. Transfer it to a well-seasoned 12-inch cast iron skillet and center it so that an even amount of crust hangs over each side. Press the crust into the bottom corners of the skillet so that it is nice and snug (you don't want any air bubbles appearing), still allowing the extra crust to hang over the side. Pour in the filling and fold the extra crust back over it. Trim the crust as necessary so that it makes about a 2-inch border around the filling. Place in the oven and bake for 45 to 55 minutes or until the crust turns golden and the filling is aromatic. Remove and allow to cool for 20 minutes before garnishing with the basil blossoms and serving.