.
In a clean bowl, stir together the remaining 1/2 cup flour and 1/2 cup sugar. Add the finely chopped rhubarb and toss until all the pieces are evenly coated and no dry pockets of flour remain. The rhubarb should look a bit powdery.
Spread the coated rhubarb mixture evenly over the warm, partially baked crust in the glass dish, making sure you reach all the corners so every bar gets some fruit.
Sprinkle the reserved crumb mixture evenly over the rhubarb layer, covering most of the fruit. This will bake into a light, buttery crumble on top.
Return the dish to the oven and bake for 30–35 minutes, or until the top is lightly golden and the rhubarb layer is bubbling up a bit around the edges and looks thick and glossy through the glass.
Remove the pan from the oven and place it on a cooling rack. Let the bars cool in the glass dish until just warm or at room temperature; this helps the filling set so you can cut neat squares.
Once cooled and set, cut into 12 bars (or smaller pieces for a potluck) and serve straight from the glass casserole dish. Store any leftovers covered at room temperature for a day, or in the refrigerator for up to 3 days.
Variations & Tips
If your rhubarb is especially tart, you can increase the sugar in the filling by 2–4 tablespoons, keeping the total ingredients the same but shifting a little more of the sugar to the rhubarb mixture and a little less in the crust. For a slightly thicker bar, use a smaller glass baking dish (such as 8×11 inches) and add 5–10 minutes to the baking time, watching so the top doesn’t get too dark. Frozen rhubarb can be used in a pinch: thaw it completely, pat it dry, then measure and proceed as directed, knowing the filling may be a touch softer. To keep the old-fashioned 4-ingredient spirit but add a hint of flavor, you can use salted butter instead of unsalted and reduce the sugar in the crust by a tablespoon or two for a more pronounced sweet-salty contrast. If you prefer a crisper base, bake the crust 5 minutes longer before adding the rhubarb. These bars travel well: cool them completely in the glass dish, cover tightly with foil, and tuck them into a towel-lined box or cooler for a picnic or potluck.
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