Icelandic Rhubarb Bars

Name: Icelandic Rhubarb Bars
Ingredients: 1½ lb. rhubarb, sliced into 1-in. lengths, divided (about 2 cups); ½ cup turbinado sugar; Pinch salt; Two 2-in. pieces of red beet, optional (see headnote); 2¾ cups all-purpose flour; 2½ cups quick oats; 2 tsp. baking soda; 1½ cups turbinado sugar; 1¼ tsp. kosher salt; 2 large eggs, at room temperature; 27 Tbsp. unsalted butter, at room temperature, cubed; 1 Tbsp. whole milk; 2 egg yolks; 1½ cups all-purpose flour; 1¼ cups light or dark brown sugar; 18 Tbsp. unsalted butter, at room temperature, cubed; 1¼ cups quick oats; 1 cup rolled oats