- Make the crust: In a food processor, pulse together flour, butter, sugar, and salt until the mixture has the consistency of fine sand. Slowly add water, 1 tablespoon at a time, and pulse a few times until dough starts to form. Do not overmix. Turn dough out onto a clean, lightly floured surface and knead until it forms a smooth ball. Refrigerate 1 to 2 hours before using.
- Make the filling: Preheat oven to 350 degrees. In a large bowl, combine rhubarb, sugar, lemon juice and zest, cardamom, and cornstarch until well incorporated. Set aside while you roll out the pastry crust.
- On a lightly floured surface, roll out dough to about a 1/8-inch thickness. Line the bottom and sides of a 2-quart baking dish (oval or rectangular) with dough. Trim dough so it’s flush with the baking dish.
- Pour rhubarb mixture into baking dish. Gather scraps into a ball and roll remaining dough to 1/8-inch thick. With a knife or a fluted pastry cutter, cut ten to twelve 1-inch strips and weave a lattice pattern over top of rhubarb. In a small bowl, whisk egg to make an egg wash. With a pastry brush, brush egg wash over lattice top.
- Place on a baking sheet and bake in the middle of oven for 1 hour and 15 minutes or until crust is golden brown.
- Make the dip: In a small bowl, dissolve cornstarch with a few tablespoons of whole milk to make a slurry. Combine remaining milk, evaporated milk, and honey in a medium saucepan over medium heat. Add the slurry and cook, stirring constantly, until the mixture thickens and coats the back of a spoon. Remove from heat and lay a piece of plastic wrap directly on surface of the sauce to prevent a skin from forming. Set aside until ready to serve with sonker, cooled to room temperature