In a medium saucepan over medium heat, combine chopped rhubarb, sugar, grated green apple, lemon juice, and water. Cook for about 35-40 minutes until it thickens.

Transfer rhubarb filling to a bowl and allow it to cool to room temperature.

In a large mixing bowl, whisk together the flour, sugar, and salt. Cut in cold butter until it resembles coarse crumbs. Add the egg and vanilla, mixing until a soft dough forms.

Divide the dough into two portions: one part for the base, and the other for the crumble topping. Wrap in plastic and refrigerate for 30 minutes.

Preheat your oven to 350°F (180°C) and line a 9-inch square baking pan with parchment paper.

Press the larger portion of dough evenly into the bottom of the prepared baking pan.

Spread the cooled rhubarb filling over the crust.

Crumble the reserved dough over the filling evenly.

Bake for 35-40 minutes, or until the crumble topping is lightly browned.

Let the bars cool completely in the pan for about an hour, then slice into squares.