Prep the pan and oven. Heat the oven to 350°F (175°C). Line an 8-inch square pan with parchment, leaving overhang for easy lifting. Lightly grease the sides.

Make the rhubarb filling. In a small saucepan, combine rhubarb, sugar, orange juice, and a pinch of salt. Cook over medium heat for 6–8 minutes, stirring, until the rhubarb softens and releases juices.

Thicken the filling. Stir the cornstarch with 1 tablespoon cold water to make a slurry. Pour it into the rhubarb, simmer 1–2 minutes until thick and jammy. Remove from heat and stir in vanilla. Let cool slightly.

Mix the dry crust ingredients. In a large bowl, whisk oats, flour, brown sugar, baking powder, cinnamon, and salt until combined.

Add the wet ingredients. In a measuring cup, whisk melted butter, egg, and vanilla. Pour into the dry mixture. Stir until clumps form and no dry flour remains. The mixture should be crumbly but moist when pressed.

Form the base. Press about two-thirds of the oat mixture firmly into the prepared pan to make an even bottom layer. Use the back of a measuring cup to compact it well.

Add the filling. Spread the rhubarb mixture evenly over the base. If using strawberries or chia seeds, sprinkle them over the rhubarb now.

Top with crumble. Scatter the remaining oat mixture evenly over the filling. Don’t press too hard—lightly pat it so it adheres. Sprinkle with turbinado sugar if you like a sparkly, crisp top.

Bake. Bake for 28–34 minutes, until the top is golden and the filling is bubbling around the edges.

Cool and slice. Cool in the pan for at least 1 hour. Lift out using the parchment and cut into 12 bars. For the cleanest cuts, chill for 30 minutes before slicing.