In a medium saucepan, combine the rhubarb, sugar, grated apple, lemon juice, water, and sea salt. Cook over medium-high heat, stirring occasionally until the mixture boils (about 10 minutes). Reduce heat and let simmer, stirring frequently until thickened (20-30 minutes). Cool to room temperature.

In a large mixing bowl, whisk together the flour, sugar, and salt. Add the softened unsalted butter, egg, and vanilla extract. Mix until a dough forms. Divide the dough into two portions, reserving three-quarters for the base and one-quarter for the topping.

Preheat your oven to 350°F (180°C). Grease a 9x9 inch baking pan. Press the larger portion of dough into the bottom of the pan, creating a smooth base. Pour the cooled rhubarb filling over the dough and spread it evenly. Crumble the remaining dough over the filling.

Bake in the preheated oven for 35-40 minutes until the top is lightly golden brown. The aroma will fill your kitchen.

Once baked, let it cool completely. Chill in the refrigerator for about an hour for easier slicing. Cut into squares and enjoy!