Make the Filling Base: Add rhubarb, sugar, apple, lemon juice, water, and salt to a saucepan. Mix it all together.
Cook Until Boiling: Heat on medium-high and stir often until it starts bubbling, about 10 minutes.
Thicken the Filling: Lower heat and keep stirring until it thickens like a jam, about 20–30 minutes.
Cool the Filling: Transfer to a bowl and let it cool completely. Chill if you want to prep ahead.
Prepare the Dough: In a bowl, mix flour, sugar, and salt.
Add Wet Ingredients: Add butter, egg, and vanilla. Mix until it forms a soft dough.
Divide the Dough: Split into ¾ portion and ¼ portion. Let it rest while prepping the pan.
Preheat the Oven: Set to 350°F (180°C).
Prep the Pan: Butter and line a 9-inch square pan with parchment paper.
Form the Base: Press the larger dough portion evenly into the bottom.
Add Filling: Spread about 1 cup of rhubarb filling over the base.
Prepare Topping: Mix the smaller dough portion with almonds or oats if using.
Top It Off: Break dough into small pieces and place over the filling.
Bake: Bake for about 40 minutes, rotating halfway, until lightly golden.
Cool Completely: Let it cool, then chill in the fridge for easier cutting.
Slice and Serve: Cut into squares. Try not to eat half before sharing.
