Step 1

Combine grapefruit zest and juice, hibiscus flowers (if using), sugar, and salt in a medium saucepan; scrape in seeds from vanilla bean and add pod. Bring to a boil over medium heat. Reduce heat and simmer, stirring occasionally, until sugar is dissolved, about 45 seconds.

Step 2

Set aside ½ cup rhubarb; add remaining rhubarb to pan and simmer, stirring often and mashing with a wooden spoon, until rhubarb is softened and mixture is thick and jammy, 10–12 minutes.

Step 3

Remove pan from heat and stir reserved rhubarb into jam. Let cool. Remove vanilla bean and hibiscus; discard. (You should have about 1½ cups jam.)

Step 4

Do ahead: Jam can be made 5 days ahead. Transfer to an airtight container; cover and chill.

Step 5

Preheat oven to 350°. Coat an 8x8” pan with nonstick spray, then line with parchment paper, leaving an overhang on 2 sides. Coat parchment. (These bars have a tendency to stick.)

Step 6

Toast nuts on a rimmed baking sheet until fragrant and slightly darkened, 5–8 minutes. Let cool, then chop very coarsely (it’s okay to cut some just in half).

Step 7

Meanwhile, cook butter in a small saucepan over medium-low heat, stirring often, until it foams, then browns, about 5 minutes. Immediately pour into a small heatproof bowl, making sure to include all of the brown bits.

Step 8

Whisk flour, oats, brown sugar, granulated sugar, salt, and baking soda in a large bowl to combine, then whisk in nuts. Drizzle in butter and, using a rubber spatula, mash into dry ingredients until a clumpy, wet mixture forms (it should hold together when squeezed in your hand). Transfer 3 cups mixture (no need to pack) to prepared pan. Using your hands, press and compact into an even layer.

Step 9

Spread jam over base, leaving a ¼” border to prevent jam at the perimeter from getting too dark, then crumble remaining nut mixture over. If you’d like bigger clumps, press the mixture together in your hands before scattering over jam.

Step 10

Bake, rotating pan halfway through, until top feels firm and is deep golden brown, 40–45 minutes. Let cool in pan, then use parchment overhang to remove. Transfer to a cutting board and slice into 16 bars. (Chilling for a couple hours first will make them even easier to slice.)

Step 11

Do ahead: Bars can be baked 5 days ahead. Transfer to an airtight container; cover and chill.