To a medium saucepan, add the chopped rhubarb, sugar, lemon zest, and salt. Bring the mixture to a boil over medium-high heat while stirring frequently. Once it reaches a boil, turn the heat down slightly but maintain a steady simmer.

Continue to cook the rhubarb for about 7-10 minutes, or until it has reduced and thickened quite a bit.

In a small bowl, whisk the cornstarch with the lemon juice to create a smooth slurry. Pour this mixture into the simmering rhubarb while stirring constantly to prevent any clumping.

Remove the pan from the heat and stir in the vanilla extract. Let the filling cool completely, about 40 minutes, before spreading it over the tart crust. I start preparing the crust/topping 30 minutes after the filling is done cooking.

In a large mixing bowl, stir together the dry ingredients (flour through salt).

Add the vanilla to the melted butter and stir into the dry mixture. Use a wooden spoon to combine it the best you can, then use your hands to bring it together. It’ll look soft and slightly clumpy. It won’t form a smooth ball, but it should hold together when pressed between your fingers.

Reserve 200 grams (a little less than 1 cup) to use for the topping.

Press the remaining mixture into the ungreased tart pan, only going about 2/3 up the sides of the ungreased tart pan.

Sprinkle the remaining topping over the top of the rhubarb mixture and very gently press down so no pointy bits are sticking up (you don’t want them to burn).

Remove the tart and baking sheet from the oven and place on a cooling rack to cool for 30 minutes or until you can easily remove the tart pan from the baking sheet without burning your hands.