Rhubarb–Brown Butter Bars

Name: Rhubarb–Brown Butter Bars
Ingredients: Zest and juice of ½ grapefruit or 1 orange; 3 dried hibiscus flowers (optional); ¾ cup (150 g) granulated sugar; ¼ tsp. kosher salt; 1 vanilla bean, split lengthwise (optional); 1 lb. rhubarb, sliced crosswise into ½”-thick pieces (about 4 cups), divided; Nonstick cooking oil spray; 1¼ cups blanched hazelnuts, raw pecans, and/or raw or blanched almonds; ¾ cup (1½ sticks) unsalted butter, cut into pieces; 1¼ cups (156 g) all-purpose flour; 1¼ cups (125 g) old-fashioned oats; ¼ cup plus 1 Tbsp. (packed; 63 g) dark brown sugar; ¼ cup (50 g) granulated sugar; 1 tsp. Diamond Crystal or ½ tsp. Morton kosher salt; ½ tsp. baking soda; Dried hibiscus flowers, also called Jamaica flowers, are available at Latin markets and some natural foods stores.